#include<bits/stdc++.h>
using namespace std;
int main(){
    int n;
    float x,g,s,b;
    cin>>n;
    g=n%10;
    s=n/10%10;
    b=n/100;
    x=(b+s)/(s+g);
    printf("%.2f",x);
}
/**************************************************************
	Problem: 1608
	User: huanglingkai
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/