#include <bits/stdc++.h>
using namespace std;
#define int long long
typedef pair<int,int> P;
int INF = 1e16+7;
int mod = 1e9+7;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
signed main() {
    vector<int>cnt(5000005,true);
    cnt[0] = cnt[1] = false;
    for(int i = 2; i <= 5000005; i++) {
        if(!cnt[i]) {
            continue;
        }
        for(int j = i*2; j <= 5000005; j+=i) {
            cnt[j] = false;
        }
    }
    int T;
    cin >> T;
    for(int i = 0; i < T; i++) {
        int A,P;
        cin >> A >> P;
        if(!cnt[P]) {
            cout << -1 << endl;
            continue;
        }
        cout << P%A << endl;
    }
}