#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,g,s,b;
	double ans;
	cin>>n;
	b=n/100;
	s=n/10 % 10;
	g=n%10;
	ans=(b+s)*1.0/(s+g);
	printf("%.2f",ans);
	
}



/**************************************************************
	Problem: 1608
	User: zhuangsongyu
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/