using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;
using System.Globalization;
using System.Runtime.CompilerServices;
using System.Runtime.Intrinsics.X86;
using System.Runtime.Intrinsics.Arm;

class Program
{
    static int NN => int.Parse(ReadLine());
    static long[] NList => ReadLine().Split().Select(long.Parse).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var (n, k) = (c[0], c[1] - 1);
        var a = NList;
        var left = 0L;
        for (var i = k; i >= 0; --i)
        {
            if (a[i] == 0) break;
            left += a[i];
            if (i != k && a[i] == 1) break;
        }
        var right = 0L;
        for (var i = k; i < n; ++i)
        {
            if (a[i] == 0) break;
            right += a[i];
            if (i != k && a[i] == 1) break;
        }
        if (a[k] > 1) WriteLine(left + right - a[k]);
        else WriteLine(Math.Max(left, right));
    }
}