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#include <iostream>
#include <set>

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

	std::set<int> seen;
	int how_many = 0;
	long long result = 0;
	for (int i = 0; i < n; i++) {
		int x;
		std::cin >> x;
		if (seen.find(x) == seen.end()) {
			result += (i - how_many);
			++how_many;
		}
		seen.insert(x);
		if (seen.size() == k) {
			std::cout << result << '\n';
			return 0;
		}
	}
	std::cout << "-1\n";
	return 0;
}