#include<iostream>
#include<iomanip>
using namespace std;
class A{
    public:
    static void display(int hr[],int task[],int n){
        double hours =0;
        double ts=0;
        for(int i=0;i<n;i++){
            hours += hr[i];
            ts+= task[i];
        }
        cout<<fixed<<steprecision(2)<<ts/hours;
    }
};
int main(){
    string name,id;
    int n;
    cin>>name>>id>>n;
    int hr[n];
    int task[n];
    for(int i=0;i<n;i++){
        cin>>hr[i];
        cin>>task[i];
        if(hr[i]<0|| task[i]|| task[i]>100){
            cout<<"Invalid input";
            return 0;
        }
    }
    A::display(hr,task,n);
}