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#include <iostream>
#include <array>
#include <algorithm>

using std::size_t;

template <typename T, size_t N>
class MyArray : std::array<T, N>
{
public:
  constexpr void resize(size_t newSize) noexcept
  {
    _size = newSize;
  }
  constexpr typename std::array<T, N>::const_iterator cend() const noexcept
  {
    return std::array<T, N>::cbegin() + _size;
  }
  using std::array<T, N>::operator[];
  using std::array<T, N>::cbegin;

private:
    size_t _size{0};
};

int n, // number of bottles 1..500 000
    k; // prefix length 1..n

MyArray<int, 500010> bottles{}; // number of the ith bottle
std::array<bool, 500010> alreadySeen; // set of already seen bottles

long long int sum = 0; // number of moves needed to bring the k different bottles to the beginning of the array

int main()
{
  std::ios::sync_with_stdio(false);

  std::cin >> n >> k;

  bottles.resize(std::size_t(n));

  for (int i = 0; i < n; i++)
    std::cin >> bottles[size_t(i)];

  auto NotInSet = [](int bottleNumber){ return !alreadySeen[size_t(bottleNumber)]; };

  auto current = bottles.cbegin();

  for (int i = 0; i < k; i++)
  {
    current = std::find_if(current, bottles.cend(), NotInSet);

    if (current == bottles.cend())
    {
      std::cout << "-1\n";
      return 0;
    }
    auto const pos = current - bottles.cbegin();

    sum += (pos - i);
    alreadySeen[size_t(*current)] = true;
    current++;
  }
  std::cout << sum << '\n';
}