#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,a,b,c;
	cin>>n;
	a=n%10;
	b=n/10%10;
	c=n/100%10;
	printf("%.2f",(a+b)/(b+c));
} 
/**************************************************************
	Problem: 1608
	User: linzihang
	Language: C++
	Result: Wrong Answer
****************************************************************/