import java.io.*;
import java.lang.*;

public class Moyenne
{
  public static void main( String[] args )
  {
    // Tampon (buffer) du flux d'entree standard (clavier)
    BufferedReader in = new BufferedReader( new InputStreamReader( System.in ) );

    int x = 0, n = 0;
    double somme = 0;

    try
    {
      String st = in.readLine( );
      x = Integer.parseInt( st );
      while ( x >= 0 )
      {
        somme += (double)x;
        n++;
        st = in.readLine( );
        x = Integer.parseInt( st );
      }
      int length = n;
      double moy = (double)somme / length;
      System.out.format( "%.2f\n", moy );
    }

    catch(Exception e) 
    {
      System.out.println("Entree invalide !!");
    }

  } // main()

} // class Moyenne