import java.math.BigInteger;
import java.util.Scanner;
public class Test01 {
 public static void main(String[] args) {
  Scanner sc =null;
  while (true){
   try {
    System.out.println("请输入一个正整数:");
    sc = new Scanner(System.in);
    int n = sc.nextInt();
    if(n<0){
     System.out.print("您输入的数据不合法,");
    }else{
     BigInteger bigInteger = factorial(n);
     System.out.println(n+"的阶乘值为:"+bigInteger);
     break;
    }
   }catch (Exception e){
    System.out.print("您输入的数据不合法,");
   }
  }
  sc.close();
 }
 public static BigInteger factorial(int n){
  if(n==0){
   BigInteger bi = new BigInteger(String.valueOf(1));
   return bi;
  }else{
   BigInteger b1 = new BigInteger(String.valueOf(n));
   BigInteger b2 = new BigInteger(String.valueOf(factorial(n-1)));
   return b1.multiply(b2);
  }
 }
}