using static System.Math;
using System;

public class Hello
{
    static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var n = int.Parse(line[0]);
        var k = int.Parse(line[1]) - 1;
        line = Console.ReadLine().Trim().Split(' ');
        var a = Array.ConvertAll(line, int.Parse);
       long ans;
        if (a[k] == 0) ans = 0;
        else if (a[k] == 1) ans = Max(getAns(n, k, a), getAns2(n, k, a)) + 1;
        else ans = a[k] + getAns(n, k, a) + getAns2(n, k, a);
        Console.WriteLine(ans);
    }
    static long getAns(int n, int k, int[] a)
    {
        var ans = 0L;
        var p = k + 1;
        while (p < n)
        {
            if (a[p] >= 2) { ans += a[p]; p++; }
            else if (a[p] == 1) { ans++; break; }
            else break;
        }
        return ans;
    }
    static long getAns2(int n, int k, int[] a)
    {
        var ans = 0L;
        var p = k - 1;
        while (p >= 0)
        {
            if (a[p] >= 2) { ans += a[p]; p--; }
            else if (a[p] == 1) { ans++; break; }
            else break;
        }
        return ans;
    }
}