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#include <iostream>
#include <set>
using namespace std;

int k, n;

int main()
{
  ios_base::sync_with_stdio(0);

  cin >> n >> k;
  
  int ans = 0;
  int tasted = 0;
  set<int> seen_brands{};
  int a = 0;

  for (int i=0; i<n; i++) {
    cin >> a;
    if (seen_brands.find(a) == seen_brands.end()) {
      ans += i - tasted;
      tasted++;
      seen_brands.insert(a);
      if (tasted == k)
        break;
  }}

  if (tasted < k)
    cout << -1;
  else
    cout << ans;

  return 0;
}