#include<iostream>
#include<algorithm>
#include<queue>
#include<utility>
#include<cmath>
#pragma warning(disable:4996)
#define inf 0x3f3f3f3f
typedef long long ll;
using namespace std;



int main()
{
    int t;
    cin >> t;
    while (t--) {
        int n;
        double max=0, a, b;
        cin >> n;
        for (int i = 0; i < n; i++) {
            scanf("%lf%lf", &a, &b);
            if ((1 - b) / (a + 1 - b) > max) {
                max = (1 - b) / (a + 1 - b);
            }
        }
        printf("%.5lf\n", max);
    }
    

    return 0;
}