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

/**************************************************************
	Problem: 1608
	User: chenjunlin
	Language: C++
	Result: Accepted
	Time:7 ms
	Memory:2072 kb
****************************************************************/