#include <iostream>
#include <cstdio>
using namespace std;
int  b, T, n;
double c;
int main() {
    cin >> T;
    while(T --){
        cin >> n;
        double ans = 0;
        for(int i = 1; i <= n; ++i){
            cin >> b >> c;
            ans = max(ans, (1 - c)/(1 + b - c));
        }
        printf("%.5lf\n", ans);
    }
    return 0;
}