#include<bits/stdc++.h>
using namespace std;
int main(){
	int n;
	float h,a,b,c;
	cin>>n;
	a=n%10;
	b=n/10%10;
	c=n/100;
	h=(c+b)/(b+a);
	printf("%.2f",h);
	
}

/**************************************************************
	Problem: 1608
	User: zhengzihao3
	Language: C++
	Result: Accepted
	Time:19 ms
	Memory:2072 kb
****************************************************************/