#include<bits/stdc++.h>

using namespace std;

typedef long long ll;

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

  cin >> N >> K;
  --K;
  for(int i = 0; i < N; i++) cin >> A[i];

  int left = 0, right = N - 1;
  for(int i = 0; i < N; i++) {
    if(i < K && A[i] <= 1) left = max(left, i);
    if(i > K && A[i] <= 1) right = min(right, i);
  }

  ll aaa = 0, bbb = 0;
  for(int i = left; i <= K; i++) aaa += A[i];
  for(int i = right; i >= K; i--) bbb += A[i];

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