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#include <iostream>
#include <vector>

using namespace std;

int main() {
	ios_base::sync_with_stdio(false);

	vector<bool> v(500000+1, false);

	int N, K;
	cin >> N >> K;

	unsigned long long penalty = 0;
	long long sum = 0;
	int cnt = 0;
	for (int i = 0; i < N; i++) {
		int a;
		cin >> a;
		if (v[a]) {
			penalty++;
		} else {
			v[a] = true;
			sum += penalty;
			cnt++;
		}
		if (cnt >= K)
			break;
	}

	if (cnt < K)
		sum = -1;

	cout << sum << endl;

	return 0;
}