#include <iostream>
#include <iomanip>
using namespace std;

int main(){
	int n,g,s,b;
	cin>>n;
	b = n / 100;
	s = n / 10 % 10;
	g = n % 10;
	
	cout<<fixed<<setprecision(2)<<(b+s)*1.0/(s+g);
}


/**************************************************************
	Problem: 1608
	User: admin
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/