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#include <stdio.h>

static int n, k;
static int t[500000];

static void dump(void)
{
	int i;

	for (i = 0; i < n; i++)
		fprintf(stderr, " %d", t[i]);
	putc('\n', stderr);
}

int main(void)
{
	int i, j;
	long long r = 0;
	static int s[500001];

	scanf("%d%d", &n, &k);
	for (i = 0; i < n; i++)
		scanf("%d", &t[i]);

	//dump();

	for (i = 0, j = k; i < k; i++) {
		int b = t[i];

		if (s[b]) {
			int b2;

			while (j < n) {
				b2 = t[j];
				if (s[b2])
					j++;
				else
					break;
			}
			if (j >= n) {
				r = -1;
				break;
			}

			r += j - i;
			t[j++] = b;
			t[i] = b = b2;
			//fprintf(stderr, "r=%lld i=%d j=%d:", r, i, j);
			//dump();
		}

		s[b]++;
	}

	//fprintf(stderr, "r=%lld i=%d j=%d:", r, i, j);
	//dump();

	printf("%lld\n", r);
	return 0;
}