Factorial Program

/**
 * Calculates and prints out the factorial.
 *
 * @author alu_pahrata
 * @version 8-27
 */
import java.util.Scanner;
public class Factorial
{
    public static void main (String[] args){
    int number = 1, multby = 1, input, counter = 0;
    Scanner scan = new Scanner(System.in);
    System.out.println("Welcome to the Factorial Program!");
    System.out.print("Please enter an integer: ");
    input = scan.nextInt();
    System.out.println("Your number is " + input + "!");
    while (input < 0){
        System.out.println("You have entered in a negative number.");
        System.out.print("Please enter in a positive number: ");
        input = scan.nextInt();
        System.out.println();
    }
    /*
     * This checks to see if a user has entered in 0, if they have in fact entered in 0 then the
     * final calculation that gets printed at the end of the program,
     * which in this case is the variable "number", will be set to 1
     * causing the final output to be "0! = 1
     */
    if (counter == input){
        number = 1;
    }
    /*
     * This while loop checks to see if counter is not equal to input
     * if they are not equal then multby and number are multiplied,
     * after that multby and counter are both increased by one
     * This while loop will keep reapeating untill counter is equal to input
     * This is the factorial algorithm.
     */
    while (counter != input){
        number = multby * number;
        multby++;
        counter++;
    }
    System.out.println(input + "! = " + number);
    }
}