# https://atcoder.jp/contests/abc186/tasks/abc186_e
from math import gcd
T = int(input())
for _ in range(T):
    N, M = map(int, input().split())
    N, S, K = 10 ** 9, M, N
    g = gcd(K, N)
    if S % g:
        print(-1)
    else:
        N //= g
        S //= g
        K //= g
        # ans = -S * K^-1 (mod N)
        ans = (N - S * pow(K, -1, N)) % N
        print(ans)