#include<bits/stdc++.h>
using namespace std;
int main(){
    int T; scanf("%d", &T);
    while (T--){
        int n; scanf("%d",&n);
        double ans=0;
        for (int i=1;i<=n;i++){
            int b; double c;
            scanf("%d%lf", &b,&c);
            ans=max(ans,(1-c)/(b-c+1));
        }
        printf("%.5f\n",ans);
    }
}