#include<iostream>
using namespace std;
class workout
{
    public;
    void display(int arr[],int n){
        double cal=0;
        double min=0
        for (int i=0;i<(n*2);i++){
            if (i%2==0){
                min+=arr[i];
            }else{
                cal+=arr[i];
            }
            
        }
        cout<<fixed<<setprecision(2)<<cal/min<<endl;
    
}
};
int main(){
string type;
string id;
string goal;
int n;
cin>>type>>id>>goal>>n;
int arr[n*2];
for (int i=0;i<(n*2);i++){
    cin>>arr[i];
    if (arr[i]<=0){
        cout<<"invalid input";
        return 0;
    }
}
workout obj;
obj.display(arr,n);
return 0;
    

    
}