#include<iostream>
using namespace std;
int main()
{
    int num;
    string name,id;
    cin>>name>>id;
    cin>>num;
    int hr[num],tk[num],s1=0,s2=0;
    double tot=0;
    for(int i=0;i<num;i++)
    {
        cin>>hr[i]>>tk[i];
        if(hr[i]>=0 && tk[i]>=0)
        {
        s1+=hr[i];
        s2+=tk[i];
        }
        else
        {
            cout<<"Invalid input";
            return 0;
        }
       
    }
    tot=s2/s1
    cout<<tot;
    
}