#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t;
  cin >> t;
  for (int cs = 1; cs <= t; cs++) {
    long long a[1000], c, d, e = 1, f, i, j, k = 0, l;
    cin >> c >> k;
    for (i = 0; i < c; i++) {
      cin >> a[i];
    }
    for (i = 1; i < c; i++) {
      if (a[i] != a[i - 1]) {
        e++;
      }
    }
    if (k == 1 && e > 1) {
      cout << -1 << endl;
    } else {
      d = 1;
      e -= k;
      while (e > 0) {
        e -= k - 1;
        d++;
      }
      cout << d << endl;
    }
  }
  return 0;
}
