/* NWERC 2008: Sample solution to "Easy Climb"
 * Author: Per Austrin
 *
 * Algorithm: dynamic programming
 *
 * Disclaimer: I have no idea whether this solution is correct or not.
 * I've tested it against a correct but slow solution on a bunch of
 * random cases, and it passes these + the actual test data.  I have
 * not yet had time to try to manually construct cases or to prove
 * that the algorithm actually works.
 */
#include <cassert>
#include <iostream>
#include <map>
#include <cmath>

using namespace std;
typedef long long ll;
const ll INF = 1LL<<50;

int N;
ll H[150], D;
map<int,ll> best[150];

ll Greedy(int a, int b, ll ha, ll hb) {
  ll lo = hb-D*(b-a-1), hi = hb+D*(b-a-1);
  ll at = ha, ret = 0, nat;
  for (int x = a+1; x != b; ++x, at = nat, lo += D, hi -= D) {
    nat = lo >? at-D >? H[x] <? at+D <? hi;
    ret += abs(H[x]-nat);
  }
  return ret;
}

ll Best(int l, ll hl) {
  if (l == N-1) return 1;
  ll &ret = best[l][hl];
  if (!ret) {
    ret = INF;
    for (int m = l+1; m < N; ++m) {
      ll nh = hl-D*(m-l) >? H[N-1]-D*(N-1-m) >? H[m] <? H[N-1]+D*(N-1-m) <? hl+D*(m-l);
      ret <?= abs(nh-H[m]) + Greedy(l, m, hl, nh) + Best(m, nh);
    }
  }
  return ret;
}

void solve() {
  scanf("%d%lld", &N, &D);
  for (int i = 0; i < N; ++i) scanf("%lld", H+i), best[i].clear();
  if (abs(H[0]-H[N-1]) > D*(N-1)) printf("impossible\n");
  else printf("%lld\n", Best(0, H[0])-1);
}

int main(void) {
  int N;
  for (scanf("%d", &N); N--; solve());
  return 0;
}