// external libraries
#include <stdio.h>

// constants
#define EXIT_CODE -1
#define MESSAGE_FOR_ENTER_NUMBER "Type the factorial number (0 to exit program)"
#define MESSAGE_FACTORIAL_RESULT "Result:"

// prototypes
int factorial(int number);


// main program
// loop to enter a number while number is <> EXIT_CODE
int main(void) {

  int number;
  int result;

  do {
    printf("%s\n", MESSAGE_FOR_ENTER_NUMBER);

    scanf("%d", &number);

    if (number >= 0) {
      result = factorial(number);

      printf("%s %d\n\n", MESSAGE_FACTORIAL_RESULT, result);
    }

  } while (number != EXIT_CODE);

  return 0;
}


// recursive factorial function
// return the factorial number from @number
int factorial(int number) {
  if (number == 0)
    return 1;

  return number * factorial(number - 1);
}