#include<bits/stdc++.h>
using namespace std;
int main(){
    int n;
    float s=0,g=0,i=0,m=0;
    cin>>n;
    s=n%10;
    g=n/10%10;
    i=n/100;
    m=(i+g)/(s+g);
    printf("%.2lf",m);
}
/**************************************************************
	Problem: 1608
	User: lijinkai
	Language: C++
	Result: Accepted
	Time:7 ms
	Memory:2072 kb
****************************************************************/