import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int k = sc.nextInt();
        int[] arr = new int[n + 2];
        for (int i = 1; i <= n; i++) {
            arr[i] = sc.nextInt();
        }
        if (arr[k] == 0) {
            System.out.println(0);
            return;
        }
        long left = 0;
        int idx = k - 1;
        while (arr[idx] > 1) {
            left += arr[idx];
            idx--;
        }
        if (arr[idx] == 1) {
            left++;
        }
        long right = 0;
        idx = k + 1;
        while (arr[idx] > 1) {
            right += arr[idx];
            idx++;
        }
        if (arr[idx] == 1) {
            right++;
        }
        if (arr[k] == 1) {
            System.out.println(Math.max(left, right) + 1);
        } else {
            System.out.println(left + right + arr[k]);
        }
    }
}