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/*
 *  Copyright (C) 2021  Paweł Widera
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details:
 *  http://www.gnu.org/licenses/gpl.html
 */
#include <iostream>
#include <algorithm>
using namespace std;


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

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

	bool seen[500001] = {false};

	int label;
	int last = 0;
	unsigned long long swaps = 0;

	for (int i = 0; i < n; ++i) {
		cin >> label;

		if (!seen[label]) {
			seen[label] = true;
			swaps += max(0, i - last++);

			if (--k == 0) {
				cout << swaps << endl;
				return 0;
			}
		}
	}

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