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#include <stdio.h>
typedef unsigned int ui;
size_t n, k;
ui *arr;
size_t *has_occured;
bool *is_used;
int main(void) {
	scanf("%lu %lu", &n, &k);
	arr = new ui[n];
	for (size_t i=0; i<n; ++i)
		scanf("%u", &arr[i]);

	size_t too_many_cnt = 0;
	has_occured = new size_t[n+1]{  };
	is_used = new bool[n+1]{  };
	for (size_t i=0; i<k; ++i) {
		if (has_occured[arr[i]])
			++too_many_cnt;

		++has_occured[arr[i]];
	}

	size_t result = 0;

	size_t found_needed_cnt = 0;
	for (size_t i=k; i<n && found_needed_cnt<too_many_cnt; ++i) {
		if (has_occured[arr[i]] || is_used[arr[i]])
			continue;

		is_used[arr[i]] = true;
		result += i-k - found_needed_cnt;
		++found_needed_cnt;
	}

	if (found_needed_cnt < too_many_cnt) {
		printf("-1\n");
		return 0;
	}

	for (size_t i=k-1; i!=-1; --i) {
		if (has_occured[arr[i]] <= 1)
			continue;

		// result += too_many_cnt + (k-1)-i;
		result += k-1+too_many_cnt - i;
		--has_occured[arr[i]];
		--too_many_cnt;
	}

	printf("%lu\n", result);

	return 0;
}