#include<cstdio>
#include<algorithm>
#include<cstring>
const int maxn = 1000006;
const int mod = 10007;
using namespace std;
int main()
{

    int t,n;
    scanf("%d",&t);
    while(t--){
        double maxx=0,c,b;
        scanf("%d",&n);
        while(n--){
            scanf("%lf%lf",&b,&c);
            if((1-c)/(b+1-c)>maxx)
                maxx=(1-c)/(b+1-c);
        }
        printf("%.5lf\n",maxx);
    }
    return 0;
}