#include<iostream>
#include<iomanip>
using namespace std;
int main()
{
    int c;
    cin>>c;
    float ansd=0,ansf=0;
    for(int i=0;i<c;i++)
    {
        float dis,fuel;
        cin>>dis>>fuel
        if(dis<=0)
        {
            cout<<"Invlid Distance";
            return 0;
        }
        if(fuel<=0)
        {
            cout<<"Invlid Fuel";
            return 0;
        }
        ansd+=dis;
        ansf+=fuel;
    }
    cout<<fixed<<setprecision(2)<<ansd/fuel;
    return 0;
}