首先要知道等差数列的公式

an = a1 + (n - 1) * d

sn = n * (a1 + an) / 2

let n = parseInt(readline())
let a1 = 2, d = 3
let an = a1 + (n - 1) * d
let sn = n * (a1 + an) / 2
console.log(sn)