#include<bits/stdc++.h>
using namespace std;

int main(){
	int t;
	cin >> t;
	for(int i = 0;i < t;i++){
	int a,b;
	cin >> a >> b;
	int c = b - a;
    if(b == a) cout << 0 << endl;
	else if(c < a) cout << -1 << endl;
	else{
		int ans = c;
		for(int j = 1;j * j <= c;j++){
			if(c % j == 0){
				if(j >= a) ans = min(ans,j);
				if(c / j >= a) ans = min(ans,c / j);
				
			}
		}
		cout << ans - a << endl;
	}
	}
}