求阶乘

package asda;
import java.util.Scanner;
public class Main
{
    public static void main(String[] args)
    {
        long  sum = 0;
        long  a[] = new long[25];
        a[0] = 1;
        for(int i = 1; i<=20; ++i)
        {
            a[i] = a[i-1]*i;
            sum += a[i];
        }
        System.out.println(sum);
    }
}