#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (n); i++)
#define repr(i, n) for (int i = (n) - 1; i >= 0; i--)
#define range(a) a.begin(), a.end()

int main() {
  int N, K; cin >> N >> K; K--;
  vector<ll> A(N); rep(i, N) cin >> A[i];
  int l = K - 1;
  int r = K + 1;
  while (l >= 1 && A[l] >= 2) l--;
  while (r <= N - 2 && A[r] >= 2) r++;
  ll sl = 0;
  for (int i = max(0, l); i <= K - 1; i++) {
    sl += A[i];
  }
  ll sr = 0;
  for (int i = min(N - 1, K + 1); i <= r; i++) {
    sr += A[i];
  }
  if (A[K] == 0) {
    cout << 0 << endl;
    return 0;
  }
  if (A[K] <= 2) {
    cout << max(sl, sr) + A[K] << endl;
  } else {
    cout << sl + sr + A[K] << endl;
  }
}