#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  k--;
  int a[n];
  int sum = 0;
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
  }
  if (a[k] == 1) sum++;
  for (int i = 1; i < n; ++i) {
    if (k - i < 0) {
      for (int j = k + i; j < n; ++j) {
        sum += a[j];
      }
      break;
    }
    if (k + i >= n) {
      for (int j = k - i; j >= 0; --j) {
        sum += a[j];
      }
      break;
    }
    if (a[k - i] == 1 && a[k + i] == 1) sum += 2;
  }
  cout << sum;
  return 0;
}
