#include <bits/stdc++.h>
using std::cin;
using std::cout;
using std::endl;
using std::set;
using std::map;
using std::min;
using std::max;
using std::vector;
using std::deque;
using std::queue;
using std::min;
using std::max;
using std::swap;
using std::string;
using std::sort;
using std::unique;
typedef long long ll;
typedef double db;
const int MOD = 998244353;
const int N = 1000005;
struct Pir
{
    ll x,y;
};
ll get_gcd(ll x,ll y) {return (y==0) ? x : get_gcd(y,x%y);}
ll power(const ll & x,const ll & mi)
{
    ll s1=1LL,s2=x,m=mi;
    while (m)
    {
        if (m&1) s1=s1*s2%MOD;
        s2=s2*s2%MOD;
        m>>=1;
    }
    return s1;
}
inline ll get_inv(ll x)
{
    return power(x,MOD - 2);
}
void solve()
{
    int n;
    db a,b;
    cin>>n;
    db mx=0;
    for (int i=1;i<=n;++i)
    {
        cin>>a>>b;
        mx=max(mx,(1.0-b)/(a+1-b));
    }
    printf("%.5f\n",mx);
    return;
}
int main()
{
    int T=1;
    cin>>T;
    while (T--) solve();
    return 0;
}