/* -*- coding: utf-8 -*-
 *
 * 1007.cc:  No.1007 コイン集め - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 200000;

/* typedef */

typedef long long ll;

/* global variables */

int as[MAX_N];
ll ass[MAX_N + 1];

/* subroutines */

/* main */

int main() {
  int n, k;
  scanf("%d%d", &n, &k);
  k--;

  for (int i = 0; i < n; i++) {
    scanf("%d", as + i);
    ass[i + 1] = ass[i] + as[i];
  }

  if (as[k] == 0)
    puts("0");
  else {
    int l, r;
    if (k > 0)
      for (l = k - 1; l > 0 && as[l] > 1; l--);
    else
      l = 0;
    if (k + 1 < n)
      for (r = k + 1; r + 1 < n && as[r] > 1; r++);
    else
      r = n - 1;
    //printf("l=%d, r=%d\n", l, r);

    if (as[k] == 1)
      printf("%lld\n", max(ass[k + 1] - ass[l], ass[r + 1] - ass[k]));
    else
      printf("%lld\n", ass[r + 1] - ass[l]);
  }
  return 0;
}