题解 | 等差数列

const rl = require("readline").createInterface({ input: process.stdin });
var iter = rl[Symbol.asyncIterator]();
const readline = async () => (await iter.next()).value;

void async function () {
    // Write your code here
    const num = await readline()
    let sum = 0
    for(let i=1; i<=num; i++) {
        sum += 2 + (i - 1) * 3
    }
    console.log(sum)
}()