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 () {
    let number = parseInt(await readline());
    let res = 2; // 等差数列的第一项是2
    let nextNum = 5; // 等差数列的第二项是5，用于计算下一项的值
    for(let i = 1; i < number; i++) {
        res += nextNum; // 累加当前项
        nextNum += 3; // 计算下一项
    }
    console.log(res); // 输出最终的总和
}()