#include <bits/stdc++.h>
using namespace std;

int main() {
  int N, K;
  cin >> N >> K;
  vector<long long> A(N + 1);
  for (int i = 1; i <= N; ++i) {
    cin >> A[i];
  }

  vector<long long> A2 = A;
  for (int i = 0; i < N; ++i) {
    A2[i + 1] += A2[i];
  }

  int left = K, right = K;
  for (int l = K - 1; l >= 1; --l) {
    if (A[l] == 0 || A[l] == 1) {
      left = l;
      break;
    }
    if (left == K && l == 1) {
      left = 1;
    }
  }

  for (int r = K + 1; r <= N; ++r) {
    if (A[r] == 0 || A[r] == 1) {
      right = r;
      break;
    }
    if (right == K && r == N) {
      right = N;
    }
  }

  long long sum_l = A[left] == 0 ? A2[K] - A2[left] : A2[K] - A2[left - 1],
            sum_r = A2[right] == 0 ? A2[right - 1] - A2[K - 1]
                                   : A2[right] - A2[K - 1];

  if (A[K] == 0) {
    cout << 0 << endl;
  } else if (A[K] == 1) {
    cout << max(sum_l, sum_r);
  } else {
    cout << sum_l + sum_r - A[K] << endl;
  }
}