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#include <cstdio>

int liczba_butelek;
int liczba_skrajnych_butelek;
int marki[500000];
int swaps = 0;

// jedna zamiana 1 sekunda


void swap(int& a, int& b) { //swap the content of a and b
	int temp;
	temp = a;
	a = b;
	b = temp;
	swaps++;
	//printf("%d vs %d\n", a, b);
}


int different_bootles[50000];
int different_bootles_size = 0;

bool is_unique(int& a) {
	for (int i = 0; i < different_bootles_size; i++) {
		if (different_bootles[i] == a) {
			return false;
		}
	}
	return true;
}


int main() {
	bool result_possible = true;

	scanf("%d %d", &liczba_butelek, &liczba_skrajnych_butelek);
	for (int i = 0; i < liczba_butelek; i++) {
		scanf("%d", &marki[i]);
	}

	for (int i = 0; i < liczba_skrajnych_butelek; i++) {
		while (!is_unique(marki[i]) && result_possible) {
			int j;
			for (j = i; j < liczba_butelek - 1; j++) {
				if (marki[j] != marki[j + 1]) {
					swap(marki[j], marki[j + 1]);
					break;
				}
			}
			if (j == liczba_butelek - 1) {
				result_possible = false;
			}
		}
		different_bootles[different_bootles_size++] = marki[i];
	}
		
	

	/*for (int i = 0; i < liczba_butelek; i++) {
		printf("%d ", marki[i]);
	}*/

	if (!result_possible) {
		printf("-1");
	}
	else {
		printf("%d", swaps);
	}

	return 0;
}