#include<bits/stdc++.h>
using namespace std;
const long long INF = 1e18;
vector<long long> factorize(long long d) {
  vector<long long> res;
  for (long long j = 2; j * j <= d; j++) {
    if (d % j == 0) {
      res.push_back(j);
      res.push_back(d / j);
      while (d % j == 0) {
        d /= j;
      }
    }
  }
  if (d != 1) res.push_back(d);
  return res;
}
int main() {
  int T;
  cin >> T;
  while (T--) {
    long long A, B;
    cin >> A >> B;
    if (A == B) {
      cout << 0 << '\n';
      continue;
    }
    long long d = B - A;
    vector<long long> c = factorize(d);
    sort(c.begin(), c.end());
    int N = c.size();
    long long ans = INF;
    for (int i = 0; i < N; i++) {
      long long a = (A + c[i] - 1) / c[i] * c[i];
      long long diff = a - A;
      long long b = B + diff;
      if (b % a == 0) {
        ans = min(ans, diff);
      }
    }
    if (ans == INF) ans = -1;
    cout << ans << '\n';
  }
}