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#include <bits/stdc++.h>
using namespace std;

using ll = long long;

ll solve(int n, int k, vector<int> a) {
  ll res = 0;
  vector<int> exists(n + 1);
  int next_free_pos = 0;

  for (int i = 0; i < n && next_free_pos < k; i++) {
    if (!exists[a[i]]) {
      res += i - next_free_pos;
      next_free_pos++;
      exists[a[i]] = 1;
    }
  }

  if (next_free_pos < k) {
    return -1;
  }
  else {
    return res;
  }
}

int main() {
  ios_base::sync_with_stdio(false);

  int n, k;
  cin >> n >> k;
  vector<int> a(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }

  cout << solve(n, k, a) << endl;
}