#![allow(unused_imports)]
#![allow(non_snake_case)]
use std::cmp::*;
use std::collections::*;
use std::io::Write;

#[allow(unused_macros)]
macro_rules! debug {
    ($($e:expr),*) => {
        #[cfg(debug_assertions)]
        $({
            let (e, mut err) = (stringify!($e), std::io::stderr());
            writeln!(err, "{} = {:?}", e, $e).unwrap()
        })*
    };
}

fn main() {
    let v = read_vec::<usize>();
    let (n, k) = (v[0], v[1] - 1);
    let a = read_vec::<i64>();
    if a[k] == 0 {
        println!("0");
        return;
    }
    let mut right = 0i64;
    for i in k + 1..n {
        right += a[i];
        if a[i] <= 1 {
            break;
        }
    }

    let mut left = 0i64;
    for i in (0..k).rev() {
        left += a[i];
        if a[i] <= 1 {
            break;
        }
    }
    if a[k] == 1 {
        println!("{}", max(left, right) + 1);
    } else {
        println!("{}", left + right + a[k]);
    }
}

fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}