#include <bits/stdc++.h>
using namespace std;
void _print(long long t) { cerr << t; }
void _print(int t) { cerr << t; }
void _print(string t) { cerr << t; }
void _print(char t) { cerr << t; }
void _print(long double t) { cerr << t; }
void _print(double t) { cerr << t; }
void _print(unsigned long long t) { cerr << t; }
template <class T, class V>
void _print(pair<T, V> p);
template <class T>
void _print(vector<T> v);
template <class T>
void _print(set<T> v);
template <class T, class V>
void _print(map<T, V> v);
template <class T>
void _print(multiset<T> v);
template <class T, class V>
void _print(pair<T, V> p) {
  cerr << "{";
  _print(p.first);
  cerr << ",";
  _print(p.second);
  cerr << "}";
}
template <class T>
void _print(vector<T> v) {
  cerr << "[ ";
  for (T i : v) {
    _print(i);
    cerr << " ";
  }
  cerr << "]";
}
template <class T>
void _print(set<T> v) {
  cerr << "[ ";
  for (T i : v) {
    _print(i);
    cerr << " ";
  }
  cerr << "]";
}
template <class T>
void _print(multiset<T> v) {
  cerr << "[ ";
  for (T i : v) {
    _print(i);
    cerr << " ";
  }
  cerr << "]";
}
template <class T, class V>
void _print(map<T, V> v) {
  cerr << "[ ";
  for (auto i : v) {
    _print(i);
    cerr << " ";
  }
  cerr << "]";
}
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
bool sortm(pair<int, int> &a, pair<int, int> &b) {
  int x = abs(a.first) + abs(a.second);
  int y = abs(b.first) + abs(b.second);
  return x < y;
}
void init() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
}
int n, k;
int dp[2011][2011];
int a[2011];
int calc(int i, int j) {
  if (j < 0) {
    return -1e8;
  }
  if (i == -1) {
    if (j == 0) return 0;
    return -1e8;
  }
  int &ans = dp[i][j];
  if (ans != -1) {
    return ans;
  }
  ans = (j == a[i]) + calc(i - 1, j);
  ans = max(ans, calc(i - 1, j - 1));
  return ans;
}
void solve() {
  cin >> n >> k;
  for (int i = 0; i < n + 10; i++) {
    for (int j = 0; j < n + 10; j++) {
      dp[i][j] = -1;
    }
  }
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    a[i] = i - a[i] + 1;
  }
  for (int i = 0; i <= n; i++) {
    if (calc(n - 1, i) >= k) {
      cout << i << "\n";
      return;
    }
  }
  cout << -1 << "\n";
}
int main() {
  init();
  int t = 1;
  cin >> t;
  while (t--) solve();
  return 0;
}
