#include<stdio.h>
int main()
{
	int a,b,c,d;
	scanf("%d",&a);
	b=a%10;
	c=a/100;
	d=a/10-c*10;
	printf("%.2f",(c+d)*1.0/(b+d));
	return 0;
}
/**************************************************************
	Problem: 1608
	User: admin
	Language: C
	Result: Accepted
	Time:12 ms
	Memory:1144 kb
****************************************************************/