#include <bits/stdc++.h>

using namespace std;

vector<int> factor;

void all_factor(int n){
    factor.clear();
    for (int i = 1; i*i <= n; i++){
        if (n % i == 0){
            factor.push_back(i);
            if (i*i != n) factor.push_back(n / i);
        }
    }
}

void solve(){

    int A, B, mi=2e9;
    cin >> A >> B;
    all_factor(B-A);

    for (auto x : factor){
        if (x >= A) mi = min(mi, x);
    }

    if (mi == 2e9) cout << -1 << '\n';
    else cout << mi-A << '\n';
}

int main(){
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);

    int T;
    cin >> T;

    while(T--) solve();

    return 0;
}