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#include <iostream>
#include <vector>
#include <set>

int main() {
	std::ios::sync_with_stdio(false);
	int n, k;
	std::cin >> n >> k;
	std::vector<int> vines;
	std::set<int> unique_to_k;
	
	int tmp;
	for (int i = 0; i < n; i++) {
		std::cin >> tmp;
		vines.push_back(tmp);
	}

	std::vector<int> to_left;
	int to_right = -1;

	int size = vines.size();
	for (int i = 0; i < k; i++) {
		// if not found
		if (unique_to_k.find(vines.at(i)) == unique_to_k.end()) {
			unique_to_k.insert(vines.at(i));
			to_left.push_back(i);
		}
		// if found
		else if (to_right == -1)
			to_right = i;
			
	}
	std::set<int> uniques = unique_to_k;
	for (int i = k; i < size; i++) {
		// if not found
		if (uniques.find(vines.at(i)) == uniques.end()) {
			uniques.insert(vines.at(i));
			to_left.push_back(i);
		}
		// if found
		else if (to_right == -1)
			to_right = i;
	}

	int swaps = 0;
	int shifts = 0;
	int to_left_it = 0;
	while (unique_to_k.size() < k && to_left_it < to_left.size()) {
		if (to_left.at(to_left_it) < to_right)
			to_left_it++;
		else {
			unique_to_k.insert(vines.at(to_left.at(to_left_it)));
			swaps += to_left.at(to_left_it) - to_right - shifts;
			shifts++;
			to_left_it++;
		}
	}
	if (unique_to_k.size() == k)
		std::cout << swaps;
	else
		std::cout << "-1";
}