#include<bits/stdc++.h>
using namespace std;
int main(){
	int n;
	float x,g,s,b;
	cin>>n;
	g=n%10;
	s=n/10%10;
	b=n/100;
	x=(b+s)/(s+g);
	printf("%.2f",x);
}
/**************************************************************
	Problem: 1608
	User: wangyiyang
	Language: C++
	Result: Accepted
	Time:13 ms
	Memory:2072 kb
****************************************************************/