/*
Среднее арифметическое
*/

import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Solution {
    public static void main(String[] args) throws Exception {
        //напишите тут ваш код
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        float number = Float.parseFloat(reader.readLine());
        float a = 0;
        int x = 0;

           while( number != -1){
               if(number == -1) {break;}
            number = Float.parseFloat(reader.readLine());
            a += number;
            x++;
            }
        System.out.println(a / x);
    }
}