To Codeforces staff mainly.

Currently, we have Java 11 and Kotlin 1.4.0 on Codeforces.
Kotlin therefore compiles under Java 8 instead of 11.
Java 9 and 11 bring some new API for BigIntegers such as public BigInteger.TWO (it was private) and BigInteger/BigDecimal sqrt() methods.
We can use them from Java, but not from Kotlin since Kotlin uses older version of Java.

Submission with this stuff on Kotlin 110180362.
Submission with the same stuff on Java 110180543.

Please, update Kotlin targeting from Java 8 to 11. It shouldn't bring any problems.
The only compilation flag you have to add is -jvm-target 11 (from the docs).