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#include <iostream>
#include <vector>

using namespace std;

int main() {
	int n, k;
	cin >> n >> k;
	vector<int> but(n);
	for (int i = 0; i < n; i++) cin >> but[i];
	vector<int> if_but(n, 0);
	int cnt = 0;
	long long ans = 0;
	for (int i = 0; i < n; i++) {
		if (if_but[but[i]]) continue;
		if_but[but[i]] = 1;
		ans += i - cnt;
		cnt++;
	}
	if (cnt < k) cout << -1;
	else cout << ans;
}