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#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <set>

using namespace std;

typedef long long int LL;
typedef vector<LL> VLL;
typedef set<LL> SLL;
#define FOR(x, b, e) for(int x = b; x < (e); ++x)
#define REP(x, n) for(int x = 0; x < (n); ++x)
#define ALL(c) (c).begin(), (c).end()
#define SIZE(x) ((int)(x).size())
#define PB push_back

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);

	LL n, k;
	VLL org;
	SLL s;

	cin >> n >> k;

	LL x;
	REP(i, n) {
		cin >> x;
		org.PB(x);
	}

	LL time = 0, elem = 0;
	REP(i, n) {
		if (!s.count(org[i])) {
			time += i - SIZE(s);
			s.insert(org[i]);
			elem++;
		}

		if (elem == k) break;
	}

	LL res = time ? time : -1;
	cout << res;

	return 0;
}