#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

vector<ll> divisors(ll n) {
  vector<ll> res;
  for (ll i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ll j = n / i;
      res.emplace_back(i);
      if (i != j) {
        res.emplace_back(j);
      }
    }
  }
  return res;
}

const ll INF = 1e18;
void solve() {
  ll a, b;
  cin >> a >> b;
  ll c = b - a;
  ll ans = INF;
  for (const ll d : divisors(c)) {
    if (a <= d)
      ans = min(ans, d - a);
  }
  if (ans == INF)
    ans = -1;
  cout << ans << '\n';
}

int main() {
  std::cin.tie(nullptr);
  std::ios_base::sync_with_stdio(false);
  int T = 1;
  cin >> T;
  for (int t = 0; t < T; t++) {
    solve();
  }
  return 0;
}