package memory.app.fact;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Scanner;

public class Main {
  public static void main(String[] args) throws IOException {
    Scanner in = new Scanner(new FileInputStream("fact.in"));
    PrintWriter out = new PrintWriter("fact.out");

    int n = in.nextInt();

    int num = n*5;
    int div = n;
    while(div/5 != 0) {
      num-=5;
      div=div/5;
    }

    out.printf("%d%n", num);

    in.close();
    out.close();
  }
}