#include<iostream>
#include<iomanip>
using namespace std;
int main()
{
    int n;
    cin>>n;
    float dist,fuel;
    double totaldistance=0.0;
    double totalfuel=0.0;
    
    for(int i=0;i<n;i++)
    {
        double distance,fuel
        cin>>distance>>fuel;
        cout<<"\n";
    

        if(distance<=0)
        {
            cout<<"Invalid input";
            return 0;
       
        }
        if(fuel<=0)
        {
            cout<<"Invalid input";
            return o;
           
        }
        totaldistance+=distance;
        totalfuel+=fuel;

    }
    double eff=totaldistance/totalfuel;
    cout<<fixed<<setprecision(2)<<eff;
    
    
    return 0;
}