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#include <iostream>
#include <set>


int main() {
	int result = 0;
	int found = 0;
	int pos = 0;
	int n, k, v;
	std::set<int> used;
	
	std::cin >> n >> k;
	
	for (int i = 0; i < n; i++) {
		std::cin >> v;

		if (found < k) {
			if (used.find(v) == used.end()) {
				used.insert(v);
				found += 1;
				result += i - pos;
				pos += 1;
			}
		}
	}

	if (found < k) {
		std::cout << -1;
	}
	else {
		std::cout << result;
	}

	return 0;
}