注意数据范围，递归函数里的x也会爆int 

#include <bits/stdc++.h>
using namespace std;
typedef long long LL;

int n; 
LL ans;

LL dfs(LL x)
{
	if(x == 1) return x;
	
	 x = x * dfs(x - 1);
}

int main()
{
	cin >> n;
	
	for(int i = 1; i <= n; i ++)
		ans += dfs(i);
	
	cout << ans;
		
    return 0;
}