#include<bits/stdc++.h>
using namespace std;
int main(){
	int i,a,b,c;
	cin>>i;
	a=i%10;
	b=i/10%10;
	c=i/100%10;
	printf("%.2f",(c+b)*1.0/(b+a)); 
}
/**************************************************************
	Problem: 1608
	User: linzihang
	Language: C++
	Result: Accepted
	Time:11 ms
	Memory:2072 kb
****************************************************************/