#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
#define DUMP(x) cout << #x << " = " << (x) << endl;
#define FOR(i, m, n) for (ll i = m; i < n; i++)
#define IFOR(i, m, n) for (ll i = n - 1; i >= m; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
#define FOREACH(x, a) for (auto&(x) : (a))
#define ALL(v) (v).begin(), (v).end()
#define SZ(x) ll(x.size())

int main() {
  ll n, k;
  cin >> n >> k;
  k--;
  vector<ll> a(n), s(n+1,0);
  REP(i,n){
    cin >> a[i];
    s[i+1] = s[i] + a[i];
  }

  if(a[k] == 0){
    cout << 0 << endl;
    return 0;
  }

  ll l, r;
  IREP(i,k){
    if(a[i] == 1){
      l = i;
      break;
    }
    if(i==0){
      l = 0;
    }
  }
  FOR(i,k+1,n){
    if(a[i] == 1){
      r = i;
      break;
    }
    if(i==n-1){
      r = n-1;
    }
  }

  if(a[k]>=2){
    cout << s[r+1] - s[l] << endl;
    return 0;
  }

  cout << max(s[k+1]-s[l], s[r+1]-s[k]) << endl;
}