#include<bits/stdc++.h>
using namespace std;
int main(){
	float a,d;
	int b,c,e;
	scanf("%d",&b);
	c=b%10;
	d=b/10%10;
	e=b/100%10;
	a=(e+d)/(d+c);
	printf("%.2f",a);
}
/**************************************************************
	Problem: 1608
	User: toughuatao1
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2076 kb
****************************************************************/