#include <cstdio>
#include <string>
#include <set>
#include <vector>
#include <algorithm>

using namespace std;
const int maxn = 200;

int n,b;
double c;

int main()
{
    int T;
    scanf("%d",&T);
    while(T--){
        scanf("%d",&n);
        double mi = -10;
        for( int i = 1; i <= n; i++ ){
            scanf("%d%lf",&b,&c);
            mi = max( mi , (1-c)/(b+1-c) );
        }  
        printf("%0.5f\n",mi );
    }
    return 0;
}