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#include <array>
#include <iostream>

std::array <bool, 500000> found;

int main()
{
	int n, k;
	std::cin >> n >> k;

	uint64_t total = 0;
	int slot = 0;
	for (int i = 0; i < n && slot < k; ++i) {
		int a;
		std::cin >> a;

		if (found[a])
			continue;
		found[a] = true;

		total += i - slot;
		++ slot;
	}
	if (slot != k)
		std::cout << "-1" << std::endl;
	else
		std::cout << total << std::endl;

	return 0;
}