def divisors(X):
   assert X >= 1
   from math import isqrt
   head = [d for d in range(1, isqrt(X) + 1) if X%d == 0]
   tail = [X//d for d in reversed(head)]
   if isqrt(X)**2 == X:
      head.pop()
   return head + tail



def solve(A, B):
   if A == B:
      return 0
   D = B-A
   for d in divisors(D):
      if d < A:
         continue
      else:
         X = d-A
         break
   else:
      return -1
   return X


T = int(input())
for _ in range(T):
   A, B = map(int, input().split())
   ans = solve(A, B)
   print(ans)