fn main() {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    let n: i64 = s.trim().parse().unwrap();
    let litter = n * 3;
    let m = if litter % 2 == 0 {
        litter / 2
    } else {
        litter / 2 + 1
    };
    println!("{}", m);
}