Cứuuuuuuuuuuuuu

Làm Bằng C++ nha!

#include <bits/stdc++.h>

using namespace std;

long long n, s = 0;
int main() {
 cin >> n;  
 for (long long i = 1; i <= n; i += 3)
 {
    s += i;
 }
 cout << s;
 return 0;
}

#include <bits/stdc++.h>
using namespace std;
#define int long long 

signed main(){
	int n;
	cin>>n;
	int ans=0;
	for(int i=1;i<=3*n-2;i+=3){
		ans+=i;
	}
	cout<<ans;
	//Author:jixght
}