#include <iostream>
#include<string>
#include<iomanip>
using namespace std ;
class fit{
    public:
    string ut,ui,fi;
    int logs;
    void gett{
        cin>>ut>>ui;
        cin>>fi>>logs;
    }
    void cal(){
        double du , ca;
        double tc = 0 , td = 0 ;
        for (int i = 0 ; i < logs ;i++){
            cin>>du>>ca;
            if(du<=0|| ca<= 0){
                cout<<"Invalid input";
                return ;
            }
            td += du ;
            tc += ca ;
        }
        double avg = tc / td ;
        cout <<fixed<<setprecision(2)<<avg;
    }
};
int main(){
    fit obj ;
    obj.gett();
    obj.cal();
}