#include <bits/stdc++.h>
using namespace std;
int main()
{
int n,b,s,g;
float d;
cin>>n;
b=n/100;
s=n/10%10;
g=n%10;
d=(b+s)/1.0/(s+g);
printf("%.2f",d);
	return 0;
}


/**************************************************************
	Problem: 1608
	User: wqc123
	Language: C++
	Result: Accepted
	Time:6 ms
	Memory:2072 kb
****************************************************************/