include<bits/stdc++.h>
using namespace std;
int main(){
    int n;
    float h,a,b,c;
    cin>>n;
    a=n%10;
    b=n/10%10;
    c=n/100;
    h=(c+b)/(b+a);
    printf("%.2f",h);
     
}
/**************************************************************
	Problem: 1608
	User: xuezihong1
	Language: C++
	Result: Compile Error
****************************************************************/