#include <bits/stdc++.h>
using namespace std;
const int mod = 1000000007;
long long pwr(long long a, long long b, long long mod) {
  if (b == 0) return 1;
  long long temp = pwr(a, b / 2, mod);
  temp = (temp * temp) % mod;
  if (b & 1) temp = (temp * a) % mod;
  return temp;
}
long long pwr(long long a, long long b) {
  if (b == 0) return 1;
  long long temp = pwr(a, b / 2);
  temp = (temp * temp);
  if (b & 1) temp = (temp * a);
  return temp;
}
long long gcd(long long a, long long b) { return b == 0 ? a : gcd(b, a % b); }
long long lcm(long long a, long long b) { return (a / gcd(a, b)) * b; }
long long modularInverse(long long a, long long m) { return pwr(a, m - 2, m); }
vector<vector<int> > identityMatrix;
vector<vector<int> > mul(const vector<vector<int> > &a,
                         const vector<vector<int> > &b) {
  int n = a.size();
  vector<vector<int> > ans(n, vector<int>(n));
  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < n; ++j) {
      for (int k = 0; k < n; ++k) {
        ans[i][j] += ((long long)a[i][k] * b[k][j]) % mod;
        ans[i][j] %= mod;
      }
    }
  }
  return ans;
}
vector<vector<int> > pwr(const vector<vector<int> > &a, long long n) {
  if (n == 0) {
    assert(false);
    return identityMatrix;
  }
  if (n == 1) return a;
  vector<vector<int> > tmp = pwr(a, n / 2);
  tmp = mul(tmp, tmp);
  if (n & 1) tmp = mul(a, tmp);
  return tmp;
}
int main() {
  int n, k;
  cin >> n >> k;
  priority_queue<int> Q;
  for (int i = 0; i < n; ++i) {
    int x;
    scanf("%d", &x);
    Q.push(x);
  }
  long long ans = 0;
  while (!Q.empty()) {
    int targetFloor = Q.top();
    Q.pop();
    int cap = k - 1;
    while (!Q.empty() && cap) {
      --cap;
      Q.pop();
    }
    ans += 2 * (targetFloor - 1);
  }
  cout << ans;
}
