#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long n, k;
  cin >> n >> k;
  vector<long long> a(n);
  for (long long &i : a) cin >> i;
  sort(a.begin(), a.end());
  a.resize(unique(a.begin(), a.end()) - a.begin());
  n = a.size();
  if (k == 1 and n > 1) {
    cout << -1 << endl;
    return;
  } else if (k >= n) {
    cout << 1 << endl;
    return;
  }
  long long ans;
  ans = 1, n -= k, k--;
  ans += (n + k - 1) / k;
  cout << ans << endl;
}
int32_t main() {
  ios_base::sync_with_stdio(false), cin.tie(nullptr), cin.tie(nullptr);
  long long t_c = 1;
  cin >> t_c;
  while (t_c--) solve();
}
