#include <bits/stdc++.h>
using namespace std;

int main() {
  int n, k;
  cin>>n>>k;
  --k;
  int a[n];
  for (int i=0; i<n; i++) cin>>a[i];
  if (a[k] == 0) {
    cout<<0<<endl;
    return 0;
  }
  long long r = 0;
  for (int i=k+1; i<n; i++) {
    r += a[i];
    if (a[i] <= 1) {
      break;
    }
  }
  long long l = 0;
  for (int i=k-1; i>=0 && a[i] > 0; i--) {
    l += a[i];
    if (a[i] <= 1) {
      break;
    }
  }
  if (a[k] == 1) {
    cout<<max(l, r) + 1<<endl;
  } else {
    cout<<l + r + a[k]<<endl;
  }
}