#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  int N, M;
  cin >> N >> M;
  vector<int> arr(N + 2);
  arr[0] = 1;
  for (int i = 0; i < N; ++i) cin >> arr[i + 1];
  arr[N + 1] = 1;
  int q = M, w = M;
  int r = 0;
  while (q > 0 || w < N + 1) {
    int s = arr[q] + arr[w];
    if (s == 2) r += s - (q == 0) - (w == N + 1) - (w == q);
    if (q > 0) q--;
    if (w < N + 1) w++;
  }
  cout << r << endl;
  return 0;
}
