B. Buttons1000 / 找规律

Posted 2021-10-06 幽殇默

https://codeforces.com/problemset/problem/268/B
n=4
3 41 431 4321
2 42
1

#include<bits/stdc++.h>
using namespace std;
typedef long long int LL; 
int main(void)
{
	int n; cin>>n;
	LL ans=0;
	for(int i=n-1,j=1;i>=1;i--,j++)
	{
		ans+=i*j;
	}
	cout<<ans+n<<endl;
}