题解 | #等差数列#

const readline = require('readline');

const rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
});
rl.on('line', function (line: string) {
  const num = parseInt(line);
  let total = 2;
  let numItem = 2;
  for (let i = 1; i < num; i++) {
    numItem += 3;
    total = total + numItem;
  }
  console.log(total);
});