#include <bits/stdc++.h>
using namespace std;
int main(){
    int a,b,c,d;
    double s;
    cin>>d;
    a=d%10;
    b=d/10%10;
    c=d/100;
    s=(c+b)*1.0/(a+b);
    printf("%.2f",s); 
    return 0;
}
/**************************************************************
	Problem: 1608
	User: chenjiahui
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/