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