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)
}()
