#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int n, k;
  cin >> n >> k;
  int answer = 0;
  int f[n];
  for (int i = 0; i < n; i++) cin >> f[i];
  sort(f, f + n);
  for (int i = n - 1; i >= 0; i -= k) answer += (2 * (f[i] - 1));
  cout << answer << endl;
  return 0;
}
