#include<iostream>
#include<iomanip>
using namespace std;
class fit{
    public:
    string type,id,goal;
    int logs;
    void input(){
        cin>>type>>id;
        cin>>goal>>logs;
    }
    void calc(){
        double dura, cal;
        double tcalc=0,tdura=0;
        for (int i = 0; i<logs; i++){
            cin>>dura>>cal;
            if(dura<= 0 || cal<=0){
                cout<<"Invalid input";
                return 0;
            }
            tdura+=dura;
            tcalc+=cal;
        }
        double avg = tcalc/dura;
        cout<<fixed<<setprecision(2)<<avg;
    }
};
int main(){
    fit obj;
    obj.input();
    obj.calc();
    return 0;
}