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

int n;

int main()
{
    int T;
    scanf("%d", &T);
    while (T--)
    {
        scanf("%d", &n);
        double ans = 0;
        for (int i = 0; i < n; i++)
        {
            int b;
            double c;
            scanf("%d%lf", &b, &c);
            double d = (1 - c) / (b + 1 - c);
            ans = max(ans, d);
            //cout << d << endl;
        }
        printf("%.5f\n", ans);
    }
    return 0;
}