#include<bits/stdc++.h>
using namespace std;
int main()
{int h,b,s,g;
float j;
cin>>h;
b=h/100;
s=h/10%10;
g=h%10;
j=(b+s)*1.0/(s+g);
printf("%.2f",j);	
return 0;}
/**************************************************************
	Problem: 1608
	User: lyy002
	Language: C++
	Result: Accepted
	Time:6 ms
	Memory:2072 kb
****************************************************************/