Behind the glitz of multimillion-dollar funding rounds that define today’s unicorns are often years of founder sacrifices and barely-there paychecks. Kirill Bigai, the cofounder and CEO of online education marketplace Preply, paid himself virtually nothing for an entire year while getting his now $1.2 billion business off the ground.

“We agreed to not pay each other salaries, because we didn’t have a lot of capital,” Bigai tells Fortune. “We wanted to really make it successful. We had jobs on the side…But the majority of our time was still spent with Preply.”

The first year of scaling the language-learning platform was extremely tight. Bigai was 26 when he created the company with cofounder Dmytro Voloshyn in 2012. Preply raised a small amount of pre-seed funding in its early days, but within a year it was nearly spent, forcing them to bootstrap the business. Keeping their costs lean, the founders decided to run their company operations out of their hometown of Kyiv, Ukraine. Keeping overhead low came at a personal cost: The founders had to work two careers to pay their bills.

In the first year, after its pre-seed funding dried up, the Ukrainian entrepreneurs earned no income from the operation. They spent 60 hours a week working on their start-up, juggling side-hustles to earn just enough to live. Bigai worked as an implementation consultant at software company Creatio, with prior engineering experience at Nokia Siemens Networks. Meanwhile, Voloshyn pursued a PHD in machine learning and AI, also moonlighting as a software developer for AB InBev. With little to spare, the founders ran a lean operation as they worked to prove the business could survive.

Language tutors and learners began flocking to the site, but it wasn’t until six months later that they afforded themselves a $100 monthly salary. Their take-home gradually ticked up over time, and after about two years into their founding journey, they were finally able to pay themselves $500 and quit their jobs to focus on Preply full-time. One decade later, their learning platform is a $1.2 billion unicorn connecting more than 150,000 tutors with learners across 180 countries and more than 90 languages.

How the CEO’s search for an English tutor became a $1.2 billion business

Like legions of founders before him, the Preply cofounder drew his business inspiration from a personal problem he knew firsthand.

As the son of engineers-turned-entrepreneurs, Bigai grew up in Ukraine’s capital, aspiring to get into business like his parents. But he knew that a “miracle” wouldn’t suddenly make him a founder—he had to get out there and do something about it. And the process of trying to learn English himself sparked the idea for Preply. Like millions of other bilingual learners, Bigai sought a tutor to improve his English skills in an internet era with limited options. He eventually found a tutor who taught him remotely on a 2000s-era communications platform, Skype, where users could video call online. The service was so new that Bigai spotted a real business opportunity: creating a central hub where language teachers and learners could find and connect with each other.

“We were really excited about the opportunity here,” Bigai says. “We thought that if only there were a platform that can connect you to the world’s best tutors, that would be just phenomenal, and we decided to build it.”

During the first three weeks, Preply registered 100 tutors through Google Forms, with Bigai cold-calling customers to join the platform. The company then quickly launched payment and messaging services between the tutors and learners, with the language-learning offerings limited to English. But as dozens of requests poured in daily, they quickly expanded their offerings. Soon, Preply users could become fluent in Spanish with a native speaker on the other side of the world, or find a French tutor to help them gear up for finals. In 2016, a $1.3 million seed funding round helped Preply transform its product; now, the platform has a vast international network and builds out products for teachers and students.

The Ukraine-based edtech startup now employs 800 full-time employees, and hosts 150,000 tutors on its platform. It has hundreds of tutors for each language offering, from Mandarin and Arabic to Greek and Norwegian. The business is also growing its base of millions of curious thinkers with non-language subjects; Preply now covers math, computer science, and chess. Earlier this year, its successes propelled it into the unicorn club.

This January, Preply raised $150 million in a Series D funding round led by WestCap, almost tripling its valuation to $1.2 billion. It brought the company’s total funding to more than $299 million; with the new cash injection, the business set out to develop AI tools and expand its engineering department, especially in New York and London. Reaching that valuation took more than a decade of experimentation—and, eventually, a realization that the company needed to double down on a problem its founders knew intimately.

“It came to us through a few tries and failures, and I think it really clicked to us when we started to focus on the problem that we deeply understood and that we can relate to,” the CEO says. “English learning was a big part of our journeys, and we understood how big this problem is, and how important it is to many individuals.”