#include <bits/stdc++.h> 

using namespace std;

int main(void)
{
    cin.tie(0);
    ios::sync_with_stdio(false);

    int n, k;
    long long int t;
    vector <long long int> v;

    cin >> n >> k;

    for (int i = 0; i < n; i++)
    {
        cin >> t;
        v.push_back(t);
    }

    k -= 1;

    if (v[k] == 0)
    {
        cout << 0 << '\n';
        return 0;
    }
    long long int R = 0;
    long long int L = 0;
    
    for (int i = k + 1; i < n; i++)
    {
        if (v[i] <= 1)
        {
            R += v[i];
            break;
        }
        R += v[i];
    }

    for (int i = k - 1; i >= 0; i--)
    {
        if (v[i] <= 1)
        {
            L += v[i];
            break;
        }
        L += v[i];
    }

    if (v[k] == 1)
    {
        cout << max(L, R) + 1 << '\n';
    }
    else
    {
        cout << L + R + v[k] << '\n';
    }
    return 0;
}