#include<iostream>
using namespace std;
int main() {
    int a;
    cin>>a;
    double a1[a*2];
    for(int i=0;i<a*2;i++){
        cin>>a1[i];
    }
	double n1=0,n2=0,n;
    for(int i=0;i<a*4;i+=2){
        n1+=a1[i]*a1[i+1];
        if(i%2!=0){
            n2+=a1[i];
        }
    }
    n=n1/n2;
    printf("%.3lf",n);
     return 0;
}

莫名就输出个inf