#include <bits/stdc++.h>
using namespace std;
void __print(int x) { cout << x; }
void __print(long x) { cout << x; }
void __print(long long x) { cout << x; }
void __print(unsigned x) { cout << x; }
void __print(unsigned long x) { cout << x; }
void __print(unsigned long long x) { cout << x; }
void __print(float x) { cout << x; }
void __print(double x) { cout << x; }
void __print(long double x) { cout << x; }
void __print(char x) { cout << '\'' << x << '\''; }
void __print(const char *x) { cout << '\"' << x << '\"'; }
void __print(const string &x) { cout << '\"' << x << '\"'; }
void __print(bool x) { cout << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cout << '{';
  __print(x.first);
  cout << ',';
  __print(x.second);
  cout << '}';
}
template <typename T>
void __print(const T &x) {
  int f = 0;
  cout << '{';
  for (auto &i : x) cout << (f++ ? "," : ""), __print(i);
  cout << "}";
}
void _print() { cout << "]" << endl; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cout << ", ";
  _print(v...);
}
const int mod = 1e9;
const int N = 3e5 + 1;
void solve() {
  int n, k;
  cin >> n >> k;
  set<int> s;
  bool ok = 0;
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    s.insert(x);
  }
  if (k == 1 && s.size() > 1) {
    cout << "-1\n";
    return;
  }
  if (s.size() == k) {
    cout << "1\n";
    return;
  }
  int ans = 1, cnt = 0;
  for (auto &it : s) {
    if (cnt == k) ans++, cnt = 1;
    if (!k) {
      cout << "-1\n";
      return;
    }
    if (cnt < k) {
      cnt++;
    }
  }
  cout << ans << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  cin >> t;
  while (t--) solve();
  return 0;
}
