using System;

public class Program

{

    public static void Main()

    {

        float mark = 0 ; // Nomer

        int count = 1 ; // Shomarndh

        float avg, sum = 0 ;

        while(mark >=0 ) {

            Console.WriteLine("Enter mark {0}", count);

            mark = float.Parse( Console.ReadLine() ) ;

            if( mark == -1) {

                break;

            }

            sum = sum + mark ;

            count++ ; //count = count + 1 ;

        }

        avg = sum / (count-1) ; // Majmooh / tedad

        Console.WriteLine("Average = {0}", avg);

    }

}