import java.util.Scanner;

public class Main {
	 
    /**
     * @param args
     */
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int g = a%10;
		int s = a/10%10;
		int b = a/100;
		double sum = (double)(b+s)/(s+g);
		System.out.println(String.format("%.2f", sum));
		
	}
	public static void printArr(int [] a){
		for (int i = 0 ;i<= a.length - 1 ;i++) {
			System.out.print( a[i] + " ");
		}
	}
}
/**************************************************************
	Problem: 1608
	User: admin
	Language: Java
	Result: Accepted
	Time:823 ms
	Memory:40596 kb
****************************************************************/