#include <bits/stdc++.h>
using namespace std;
template <typename T>
void printArray(T arr, int l) {
  for (int i = 0; i < l; i++) cout << arr[i] << " ";
  cout << endl;
}
int main(void) {
  int n, k;
  cin >> n >> k;
  vector<int> floors;
  for (int i = 0; i < n; i++) {
    int temp;
    cin >> temp;
    floors.push_back(temp);
  }
  sort(floors.begin(), floors.end(), greater<int>());
  int done = 0;
  long long int cost = 0;
  while (done < n) {
    cost += 2 * (floors[done] - 1);
    done += k;
  }
  cout << cost << endl;
  return 0;
}
