题解 | #等差数列#

#include <iostream>
using namespace std;
int main()
{
    //input
    int input;
    int start = 2;
    int d = 3;
    while(cin>>input)
    {
        cout << input * start + d*input*(input-1)/2 << endl;

    }
    //return
    return 0;
}