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

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];
  set<int> s;
  int64_t c = 0;
  for (int i=0; i<n; ++i) {
    if (!s.count(a[i])) {
      c += (i - s.size());
      s.insert(a[i]);
      if (s.size() == k)
        break;
    }
  }
  cout << (s.size() == k ? c : -1) << endl;
}