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

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	int n, k;
	cin >> n >> k;
	vector <int> a(n);
	vector <int> wys(n);
	int ile = 0;
	int x = n + 1;
	for (int i = 0; i < n; i++) {
		cin >> a[i];
		a[i]--;
	}
	for (int i = 0; i < n; i++) {
		if (wys[a[i]] == 0) {
			ile++;
			wys[a[i]] = i + 1;
		}
		if (ile == k) {
			x = i;
			break;
		}
	}
	if (x == n + 1) {
		cout << "-1\n";
		return 0;
	}
	long long res = 0, w = 0;
	for (int i = x; i >= 0; i--) {
		if (wys[a[i]] != i + 1)  
			res += w;
		else
			w++;
	}
	cout << res << "\n";
	return 0;
}