#include <bits/stdc++.h>
using namespace std;
void itval(istream_iterator<string> it) {}
template <typename T, typename... Args>
void itval(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << " = " << a << endl;
  itval(++it, args...);
}
const long long int MOD = 1e9 + 7;
template <typename T>
inline void print(T x) {
  cout << x << "\n";
}
template <typename T>
inline void printvec(T x) {
  for (auto a : x) cout << a << ' ';
  cout << '\n';
}
struct custom {
  bool operator()(const pair<pair<long long int, long long int>,
                             pair<long long int, long long int> > &p1,
                  const pair<pair<long long int, long long int>,
                             pair<long long int, long long int> > &p2) const {
    pair<long long int, long long int> p = p1.first, q = p2.first;
    return p.first * q.second > p.second * q.first;
  }
};
long long int get_pow(long long int a, long long int b, long long int M = MOD) {
  long long int res = 1;
  while (b) {
    if (b & 1) res = (res * a) % M;
    a = (a * a) % M;
    b >>= 1;
  }
  return res;
}
const long long int N = 5e5 + 5, inf = 2e18;
vector<long long> v;
long long int n, m;
long long int calc(long long int pos) {
  auto it = lower_bound(v.begin(), v.end(), pos) - v.begin();
  long long int up = it;
  while (up < n && v[up] == pos) up++;
  long long int cnt = 0, ab = 0, bel = 0, d2 = n - 1, d1 = 0;
  while (d2 >= up) {
    cnt += abs(v[d2] - pos) * 2;
    d2 -= m;
  }
  while (d1 < it) {
    cnt += abs(v[d1] - pos) * 2;
    d1 += m;
  }
  return cnt;
}
long long int Ternary_search(long long int l, long long int r) {
  long long int lo = l, hi = r, m1, m2;
  while (lo < hi) {
    m1 = lo + (hi - lo) / 3;
    m2 = hi - (hi - lo) / 3;
    if (calc(m1) <= calc(m2)) {
      hi = m2 - 1;
    } else {
      lo = m1 + 1;
    }
  }
  {
    string _s = "lo";
    replace(_s.begin(), _s.end(), ',', ' ');
    stringstream _ss(_s);
    istream_iterator<string> _it(_ss);
    itval(_it, lo);
  }
  return calc(lo);
}
void solve() {
  cin >> n >> m;
  v.resize(n);
  for (long long int i = (long long int)0; i < (long long int)(n); i++)
    cin >> v[i];
  sort(v.begin(), v.end());
  cout << Ternary_search(-1e9, 1e9) << '\n';
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int test = 1;
  clock_t z = clock();
  for (long long int tes = (long long int)0; tes < (long long int)(test);
       tes++) {
    solve();
  }
  fprintf(stderr, "Total Time:%.4f\n", (double)(clock() - z) / CLOCKS_PER_SEC),
      fflush(stderr);
  return 0;
}
