#include <bits/stdc++.h>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;
typedef unsigned long long ull;


int main(){
  int N, K;
  cin >> N >> K;
  vector<int> v;
  rep(i,N){
    int a;
    cin >> a;
    v.push_back(a);
  }
  K--;

  ll lhs = 0, rhs = 0;
  {
    int i = K-1;
    while(i>=0){
      if(v[i] >= 2){
        lhs += v[i];
      }
      else if(v[i] == 1){
        lhs += 1;
        break;
      }
      else {
        break;
      }
      i--;
    }
  }  
  {
    int i = K+1;
    while(i<N){
      if(v[i] >= 2){
        rhs += v[i];
      }
      else if(v[i] == 1){
        rhs += 1;
        break;
      }
      else {
        break;
      }
      i++;
    }
  }

  if(v[K] == 0){
    cout << 0 << endl;
  }
  else if(v[K] == 1){
    cout << max(lhs, rhs) + 1 << endl;
  }
  else {
    cout << lhs + rhs + v[K] << endl;
  }
  
  return 0;
}