#include<bits/stdc++.h>
using namespace std;
int main()
{
    int a;
    float d;
    cin >> a;
    int bai = a / 100;   
    int shi = (a / 10) % 10; 
    int ge = a % 10;    
    d = (float)(bai + shi) / (shi + ge);
    printf("%.2f", d);
    return 0;
}
/**************************************************************
	Problem: 1608
	User: fuhoubin
	Language: C++
	Result: Accepted
	Time:8 ms
	Memory:2072 kb
****************************************************************/