#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,a,b,c;
	float x;
	cin>>n;
	a=n%10;
	b=n/10%10;
	c=n/100;
	x=((c+b)*1.0)/(b+a);
	printf("%.2f",x);
	return 0;	
}
/**************************************************************
	Problem: 1608
	User: tangkaixuan
	Language: C++
	Result: Accepted
	Time:10 ms
	Memory:2072 kb
****************************************************************/