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#include <cstdio>

#define MAX_N 500001

int first_a[MAX_N];
int kolejnosc[MAX_N];

int main() {
	int n, k;
	scanf("%d %d", &n, &k);

	int ai;
	int x = 1;
	for (int i = 1; i <= n; i++) {
		scanf("%d", &ai);

		if (first_a[ai] == 0) {
			first_a[ai] = i;
			kolejnosc[x++] = ai;
		}
	}

	/*
	for (int i = 1; i <= 3; i++) {
		printf("%d ", kolejnosc[i]);
	}
	printf("\n");
	for (int i = 1; i <= 3; i++) {
		printf("%d ", first_a[i]);
	}
	*/

	long long int result = 0L;

	if (kolejnosc[k] == 0) {
		result = -1;

	} else {
		for (int i = 1; i <= k; i++) {
			result += first_a[kolejnosc[i]] - i;
		}
	}

	printf("%lld", result);

	return 0;
}