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

using namespace std;

const int INF = 1001001001;

void solve() {
    int a, b;
    cin >> a >> b;
    
    if (b%a == 0) {
        puts("0");
        return;
    }
    
    int d = b-a;
    int ans = INF;
    auto f = [&](int y) {
        int x = y-a;
        if (x >= 0) ans = min(ans, x); 
    };
    for (int i = 1; i*i <= d; ++i) {
        if (d%i == 0) {
            f(i);
            f(d/i);
        } 
    }
    if (ans == INF) ans = -1;
    cout << ans << '\n';
}

int main() {
    int t;
    cin >> t;
    
    while (t--) solve();
    
    return 0;
}