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