#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,x=0,y=0,z=0;
	float j;
	cin>>n;
	x=n%10;
	y=n/10%10;
	z=n/100;
	j=(z+y)*1.0/(y+x);
	printf("%.2f",j);
}
/**************************************************************
	Problem: 1608
	User: linmiaoling
	Language: C++
	Result: Accepted
	Time:14 ms
	Memory:2072 kb
****************************************************************/