use std::io;
use std::io::prelude::*;

fn main() {
    let stdin = io::stdin();
    let mut stdin_it = stdin.lock().lines();

    let k = stdin_it.next().unwrap().unwrap().split_whitespace().map(|x| x.parse::<usize>().unwrap()).last().unwrap();
    let costs: Vec<_> = stdin_it.next().unwrap().unwrap().split_whitespace().map(|x| x.parse::<u64>().unwrap()).collect();
    let state = vec![true; costs.len()];

    let mut i = k;
    let mut ans = 0;
    while i < costs.len() {
        ans += costs[i];
        i += 1 + k * 2;
    }

    println!("{}", ans);
}