//  Created by www.dida100.com on 15/11/9.
//  Copyright © 2015年 滴答编程. All rights reserved.
//
 
#include <stdio.h>
 
int main()
{
    int i;
    long double sum,mix;
    sum=0,mix=1;
    for(i=1;i<=20;i++)
    {
        mix=mix*i;
        sum=sum+mix;
    }  
    printf("%Lf\n",sum);  
}