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/*
	Zadanie: Oranżada
	Autor: Tomasz Kwiatkowski
*/

#include <bits/stdc++.h>

#define fi first
#define se second
#define pb push_back

using namespace std;
typedef long long ll;

const int MAXN = 1000*1000 + 7;
const int INF = 1000*1000*1000 + 7;

int tab[MAXN];

int main()
{
	ios_base::sync_with_stdio(0), cin.tie(0);
	
	int n, k;
	cin >> n >> k;
	vector<int> brands(n);
	for (auto& b : brands)
		cin >> b;

	bool ok = false;
	ll ans = 0;
	int pos = 0;
	vector<int> cnt(n + 1);
	for (int i = 0; i < n; ++i) {
		++cnt[brands[i]];
		if (cnt[brands[i]] == 1) {
			ans += (i - pos);
			++pos;
		}
		if (pos >= k) {
			ok = true;
			break;
		}
	}
	
	if (ok)
		cout << ans << '\n';
	else
		cout << -1 << '\n';
	return 0;
}