// 2019/12/28(土)
// 階乗 再帰

import java.util.Scanner;

public class FactRec {
    public static void main(String[] args) {
        int lastNum;
        Scanner scanner = new Scanner(System.in);
        while (true) {
            System.out.println("階乗を求めます　0から20までの間の整数を入力して下さい");
            lastNum = scanner.nextInt();
            if (lastNum < 0 || lastNum > 20) {
                System.out.println("0から20までの間の整数を入力");
            } else {
                break;
            }
        }
        long result = factrial(lastNum);
        System.out.println(lastNum + "の階乗は" + result);
    }
    public static long factrial(long lastNum) {
        if (lastNum == 0) {
            return 1;
        } else if (lastNum >=1 && lastNum <= 20) {
            return lastNum * factrial(lastNum - 1);
        } else /* 誤入力用 */ {
            return 0;
        }
    }
}