题解 | #等差数列#

use std::io;
fn main() {
    let mut s = String::new();
    io::stdin().read_line(&mut s).expect("Failed to read line");
    let n = s.trim().parse::<f32>().unwrap_or(2.0);
    println!("{}", (2.0 + (3.0 * n - 1.0)) * n / 2.0);
}