#include<bits/stdc++.h>
using namespace std;
typedef long long int uli;
typedef long double ld;
const int mx=55;
struct frac{
  uli n,d;
  bool operator <(frac f)const{
    return n*f.d<d*f.n;
  }
  bool operator <=(frac f)const{
    return n*f.d<=d*f.n;
  }
};
uli x[mx],v[mx];
uli abso(uli x){ return x<0?-x:x; }
int n;
bool can(frac t){
  int dir=0;
  for(int i=1;i<n;i++){
    if(dir==0){
      if(v[i]<=v[i-1])continue;
      frac ti={x[i]-x[i-1],v[i]-v[i-1]};
      if(ti<t)dir=1;
    }
    else{
      frac ti={x[i]-x[i-1],v[i]+v[i-1]};
      if(t<=ti){
        dir=0;
        continue;
      }
      if(v[i]>=v[i-1])continue;
      ti={x[i]-x[i-1],v[i-1]-v[i]};
      if(ti<t)return false;
    }
  }
  return true;
}
int main(){  
  int t;
  cin>>t;
  for(int tt=1;tt<=t;tt++){
    cin>>n;
    vector<pair<int,int> >xv(n);
    for(int i=0;i<n;i++)cin>>xv[i].first>>xv[i].second;
    if(n==1){
      puts("-1");
      continue;
    }
    sort(xv.begin(),xv.end());
    for(int i=0;i<n;i++)x[i]=xv[i].first,v[i]=xv[i].second;
    vector<frac>all;
    for(int i=1;i<n;i++){
      all.push_back({x[i]-x[i-1],v[i]+v[i-1]});
      if(v[i]!=v[i-1])all.push_back({x[i]-x[i-1],abso(v[i]-v[i-1])});
    }
    sort(all.begin(),all.end());
    int li=0,ls=int(all.size())-1;
    frac t=all.back();
    t.n+=t.d;
    if(can(t)){
      puts("-1");
      continue;
    }
    frac ans={-1,-1};
    while(li<=ls){
      int mid=(li+ls)>>1;
      if(can(all[mid]))ans=all[mid],li=mid+1;
      else ls=mid-1;
    }
    ld res=ld(ans.n)/ld(ans.d);
    cout<<setprecision(15)<<fixed<<res<<endl;
  }
  return 0;
}