package com.javarush.task.task05.task0507;

/*
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));
        double sum = 0;
        double cycle = 0;

        for (; ; ) {
            String x1 = reader.readLine();
            double n = Double.parseDouble(x1);

            cycle++;

            if (n == -1) {
                System.out.println( (sum) / cycle);
                break;

            }
            sum += n;
        }


    }
}