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#include <iostream>
using namespace std;

int main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);

	const int MAX_SHELF_CAPACITY = 5e5;

	int n, k;
	int brand;
	int total_brands = 0;
	unsigned long long switches = 0;

	// virgin = first_occurence
	bool was_virgin[MAX_SHELF_CAPACITY] = {};
	bool is_virgin[MAX_SHELF_CAPACITY] = {};

	cin >> n >> k;

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

		if (!was_virgin[brand]) {
			total_brands++;

			was_virgin[brand] = true;
			is_virgin[i] = true;
		}
	}

	// Answer is negative only if there are not enough brands to cover all positions.
	if (total_brands < k) {
		cout << -1;
		return 0;
	}


	int next_position = k - 1;

	for (int i = 0; i < k; i++) {
		if (!is_virgin[i]) {
			next_position++;
			while (!is_virgin[next_position]) {
				next_position++;
			}

			switches += (unsigned long long)next_position - i;
		}
	}
	
	cout << switches;
	return 0;
}