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#if defined(EMBE_DEBUG) && !defined(NDEBUG)
#include "embe-debug.hpp"
#else
#define LOG_INDENT(...) do {} while (false)
#define LOG(...) do {} while (false)
#define DUMP(...) do {} while (false)
#endif

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

namespace {
}

int main()
{
  iostream::sync_with_stdio(false);
  cin.tie(nullptr);

  int n, k;
  cin >> n >> k;

  set<int> done;
  long long res = 0;
  int j = 0;
  for (int i = 0; i < n; ++i) {
    int x;
    cin >> x;
    if (!done.insert(x).second) continue;
    if (j < k) res += i - j;
    ++j;
  }

  if (j < k) cout << -1 << endl;
  else cout << res << endl;

  return 0;
}