#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n = 0, a = 0;
  cin >> n >> a;
  long long int ar[n];
  long long int i = 0;
  for (i = 0; i < n; i++) cin >> ar[i];
  long long int pos = a - 1;
  if (n == 1) {
    if (ar[0] == 1)
      cout << "1" << endl;
    else
      cout << "0" << endl;
  } else if (n == 2) {
    if ((ar[0] == 1) && (ar[1] == 1))
      cout << "2" << endl;
    else if ((ar[0] == 1) || (ar[1] == 1))
      cout << "1" << endl;
    else
      cout << "0" << endl;
  } else {
    long long int low = pos - 1;
    long long int high = pos + 1;
    long long int ans = 0;
    if (ar[pos] == 1) ans++;
    while ((low >= 0) || (high < n)) {
      long long int l1 = ar[low];
      long long int l2 = ar[high];
      if ((low >= 0) && (high < n) && (l1 == 1) && (l2 == 1))
        ans += 2;
      else if ((low >= 0) && (high >= n) && (l1 == 1))
        ans++;
      else if ((low < 0) && (high < n) && (l2 == 1))
        ans++;
      else
        ans = ans;
      low--;
      high++;
    }
    cout << ans << endl;
  }
}
