题解 | #等差数列#

#include <iostream>
using namespace std;

int main() {
    int n;
    cin >> n;
    int a = 2;     //首项
    int sum = 2;   //求和
    for (int i = 1;i < n;i++)
    {
        a += 3;
        sum += a;
    }
    cout << sum;
}
// 64 位输出请用 printf("%lld")