#include<bits/stdc++.h>

using namespace std;

typedef long long int64;

int main()
{
  int N, K, A[200000];

  cin >> N >> K;
  --K;
  for(int i = 0; i < N; i++) cin >> A[i];
  int p = K, q = K;
  while(p > 0 && A[p - 1] >= 2) --p;
  while(q + 1 < N && A[q + 1] >= 2) ++q;
  p = max(p - 1, 0);
  q = min(q + 1, N - 1);

  int64 sum1 = 0, sum2 = 0;
  for(int i = p; i < K; i++) sum1 += A[i];
  for(int i = K + 1; i <= q; i++) sum2 += A[i];

  if(A[K] == 0) {
    cout << 0 << endl;
  } else if(A[K] == 1) {
    cout << max(sum1, sum2) + 1 << endl;
  } else {
    cout << sum1 + sum2 + A[K] << endl;
  }

}