#include <bits/stdc++.h>
using namespace std;
int dx[8] = {1, -1, 0, 0, 1, -1, 1, -1};
int dy[8] = {0, 0, -1, 1, 1, -1, -1, 1};
int n, k, f;
vector<int> v;
int main(void) {
  scanf("%d%d", &n, &k);
  for (int i = 0; i < n; i++) {
    scanf("%d", &f);
    v.push_back(f);
  }
  long long ans = 0;
  sort(v.rbegin(), v.rend());
  int to = 0;
  for (int i = 0; i < v.size(); i++) {
    to++;
    if (to == k || i + 1 == v.size()) {
      ans += 2 * (v[i - to + 1] - 1);
      to = 0;
    }
  }
  cout << ans << endl;
  return 0;
}
