#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios::sync_with_stdio(false); cin.tie(0);
  int n, k;
  cin >> n >> k;
  k--;
  vector<int> a(n);
  for (int i = 0; i < n; i++) cin >> a[i];
  long long left = 0;
  for (int j = k - 1; j >= 0; j--) {
    left += a[j];
    if (a[j] <= 1) break;
  }
  long long right = 0;
  for (int j = k + 1; j < n; j++) {
    right += a[j];
    if (a[j] <= 1) break;
  }
  if (a[k] > 1) {
    cout << left + right + a[k] << endl;
  } else if (a[k] == 1) {
    cout << max(left, right) + 1 << endl;
  } else {
    cout << 0 << endl;
  }
  return 0;
}