#include <bits/stdc++.h>
using namespace std;
int main () {
	int T;
	cin >> T;
	while (T--) {
		int a, b;
		cin >> a >> b;
		if (b % a == 0) {
			puts("0");
		} else {
			int d = b - a;
			int p = d;
			for (int x = 2; x * x <= d; x ++) {
				if (d % x == 0) {
					if (a <= x) {
						p = x;
						break;
					}
					if (a <= d / x) {
						p = d / x;
					}
				}
			}
			cout << max(-1, p - a) << endl;
		}
	}
}