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#include <cstdio>
#include <cstdlib>
#include <cstdint>
#include <vector>
#include <map>
#include <algorithm>
#include <set>
#include <time.h>
#include <queue>

using namespace std;

int main () {
	int k, n;
	scanf("%d %d", &n, &k);
	vector<int> a;
	vector<bool> ok(100000000, true);
	for (int i = 0; i < n; ++i)	{
		int x;
		scanf("%d", &x);
		a.push_back(x);
	}
	int next = 0;
	int search = 0;
	int64_t res = 0;
	while (1) {
		if (search >= n) {
			printf("-1\n");
			return 0;
		}
		if (next >= k) {
			printf("%lld\n", res);
			return 0;
		}
		if (ok[a[search]]) {
			ok[a[search]] = false;
			res += search - next;
			++next;
		}
		else {
			++search;
		}
	}
	return 0;
}