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#include <bits/stdc++.h>
using namespace std;

const int N = 1e6 + 5;

#define st first
#define nd second

typedef pair<int,int> pun;
typedef long long ll;

int main() {
	ios_base::sync_with_stdio(false);
	int n, k;
	cin >> n >> k;
	vector<int> v;
	for (int i = 0; i < n; i ++) {
		int x;
		cin >> x;
		v.push_back(x);
	}
	long long res = 0;
	vector<int> marki;
	marki.resize(n+1);
	int cnt = 0;
	for (int i = 0; i < n; i ++) {
		if (marki[v[i]] == 0 && cnt < k) {
			res += i - cnt;
			cnt ++;
		}
		marki[v[i]] ++;
	}
	if (cnt < k) {
		cout << "-1\n";
	}
	else {
		cout << res << "\n";
	}
}