#include <iostream>
#include <algorithm>
#include <queue>
#include <stack>
#include <cstring>
#include <map>
#include <string>
#include <cmath>

using namespace std;

double vremeist(int x1, int x2, int v1, int v2, int dir)
{
    if(v1<=v2 && dir==0)
        return 1000000006.0;
    if(v1>=v2 && dir==1)
        return 1000000006.0;

    return (double)(x1-x2) / (v1-v2);
}
double vremesprotivno(int x1, int x2, int v1, int v2)
{
    return (double)(x1-x2) / (v1+v2);
}
int main()
{
    int t;
    cin>>t;

    while(t--)
    {
        cout.precision(6);
        int n;
        cin>>n;

        pair<int, int> p[n+6];
        for(int i=0; i<n; i++)
        {
            cin>>p[i].first>>p[i].second;
        }
        sort(p, p+n);

        double low = 0.0;
        double high = 1000000004.2;
        double mid;
        bool f;
        int dir[n + 2];
        double maxi;

        while(fabs(low-high)>0.000001)
        {
            mid = (low+high) / 2.0;

            //            cout<<fixed<<low<<"\t"<<mid<<"\t"<<high<<endl;
            dir[0] = 0;
            f = true;
            for(int i=1; i<n; i++)
            {
                if(dir[i-1]==0)
                {
                    if(vremeist(p[i].first, p[i-1].first, p[i].second, p[i-1].second, 0)>=mid)
                        dir[i] = 0;
                    else //if(dir[i-1]==0 && vremesprotivno(p[i].first, p[i-1].first, p[i].second, p[i-1].second)>mid)
                        dir[i] = 1;
                }
                else if(dir[i-1] == 1)
                {
                    if(vremesprotivno(p[i].first, p[i-1].first, p[i].second, p[i-1].second)>=mid)
                        dir[i] = 0;
                    else if(vremeist(p[i].first, p[i-1].first, p[i].second, p[i-1].second, 1)>=mid)
                        dir[i] = 1;
                    else
                    {
                        f = false;
                        break;
                    }
                }
            }

            if(!f)
            {
                high = mid;
            }
            else
            {
                maxi = mid;
                low = mid;
            }
        }

        if(maxi > 1000000002.0)
            cout<<"-1"<<endl;
        else
            cout<<fixed<<maxi<<endl;
    }
    return 0;
}