In updating: "vx = (avx * 1ll * (vx + 10) + bvx) % 21 - 10" make sure you use real value of vx, not that divided by gcd(vx, vy);

What is test 15?
What meaning in this problem "mod"? -1 mod 10 = 9 or -1 mod 10 = 10 ?

OK. Accepted. a "mod" b = r, where a=bq+r, 0<=r<b

Edited by author 16.05.2018 01:18

Any hints why? At the end of the contest I send my dummy solution (direct simulation) and it gives WA...