#include<iostream>
#include<iomanip>
using namespace std;
int main()
{
    int n;
    cin>>n;
    double d,f,td=0,tf=0;
    while(n--)
    {
        cin>>d>>F;
        
        if(d<=0) return cout<<"Invalid Distance",0;
        if(f<=0) return cout<<"Invalid fuel",0;
        td+=d;tf+=f;
    }
    cout<<fixed<<setprecision(2)<<td/tf;
}