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#include <iostream>
#include <set>

using namespace std;

int main() {
  int n, k, setBottles = 0, bottleBrand;
  set<int> setBottlesBrands;
  long long result = 0;
  cin >> n >> k;
  for (int position = 0; position < n; position++) {
    cin >> bottleBrand;
    if (setBottles < k && setBottlesBrands.find(bottleBrand) == setBottlesBrands.end()) {
      result += position - setBottles;
      setBottles++;
      setBottlesBrands.insert(bottleBrand);
    }
  }
  if (setBottles < k)
    cout << -1;
  else
    cout << result;
  return 0;
}