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#include <iostream>
#include <string>
#include <vector>
#include <set>

using namespace std;

int main()
{
  int n, k, e;
  cin >> n >> k;
  set<int> s;
  int current_index = 0;
  long moves = 0;
  for(int i = 0; i < n; i++) {
    cin >> e;
    if (s.find(e) == s.end()) {
      s.emplace(e);
      moves += (i-current_index);
      current_index++;
      if (current_index == k) {
        cout << moves << endl;
        return 0;
      }
    }
  }

  cout << -1 << endl;
  return 0;
}