#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)
#define FOR(i,b,e) for (int i=(int)(b); i<(int)(e); i++)
#define ALL(x) (x).begin(), (x).end()

const double PI = acos(-1);

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  int n, k;
  cin >> n >> k;
  --k;

  vector<int> a(n);
  REP (i, n) cin >> a[i];

  if (a[k] < 2)
    cout << a[k] << endl;
  else {
    long long l = 0, r = 0;
    // to left
    for (int i = k-1; i >= 0; i--) {
      if (a[i] >= 2)
        l += a[i];
      else {
        if (a[i] == 1)
          ++l;
        break;
      }
    }
    // to right
    for (int i = k+1; i < n; i++) {
      if (a[i] >= 2)
        r += a[i];
      else {
        if (a[i] == 1)
          ++r;
        break;
      }
    }

    long long ret = a[k];
    if (a[k] == 2)
      ret += max(l, r);
    else
      ret += l + r;
    cout << ret << endl;
  } 
  
  return 0;
}