proc modInv(a0, b0: int): int =
var (a, b, x0) = (a0, b0, 0)
result = 1
if b == 1: return
while a > 1:
result = result - (a div b) * x0
a = a mod b
swap a, b
swap x0, result
if result < 0: result += b0
echo modInv(42, 2017)