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

int main() 
{
	ios_base::sync_with_stdio(false);
	int n, k;
	cin >> n >> k;
	vector <int> A(n);
	for (int i = 0; i < n; i++) {
		cin >> A[i];
	}
	vector <bool> First(n, false);
	unordered_set <int> S;
	int i = 0;
	for ( ; i < n; i++) {
		if (S.find(A[i]) == S.end()) {
			First[i] = true;
			S.insert(A[i]);
			if (S.size() == k) {
				break;
			}
		}
	}
	if (i == n) {
		cout << -1 <<endl;
		return 0;
	}
	long long ans = 0;
	int duplicates_at_the_end = 0;
	for (int j = i; j >= 0; j--) {
		if (!First[j]) {
			ans += (i - duplicates_at_the_end) - j;
			duplicates_at_the_end++;
		}
	}
	
	cout <<ans <<endl;
	return 0;
}