#include <iostream>
#include <vector>
using namespace std;
using ll = long long;

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

  int N, K; cin >> N >> K; K--;
  vector<int> A(N);
  for (int i = 0; i < N; i++) cin >> A[i];

  ll left = 0;
  for (int i = K-1; i >= 0; i--) {
    if (A[i] >= 2) left += A[i];
    else {
      left += A[i];
      break;
    }
  }

  ll right = 0;
  for (int i = K+1; i < N; i++) {
    if (A[i] >= 2) right += A[i];
    else {
      right += A[i];
      break;
    }
  }

  ll ans;
  if (A[K] == 0) ans = 0;
  else if (A[K] == 1) ans = A[K] + max(left, right);
  else ans = A[K] + left + right;
  cout << ans << "\n";
}