#include<bits/stdc++.h>
using namespace std;
int main(){
    int n,a,b,c;
    float s;
    cin>>n;
    a=n%10;
    b=n/10%10;
    c=n/100;
    s=(c+b)/1.0/(b+a);
    printf("%.2f",s);
    
    
    
    
    
	
	
}
/**************************************************************
	Problem: 1608
	User: lvrenxiang
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/