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#include "bits/stdc++.h"

using namespace std;

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);

	int n, k;
	cin >> n >> k;

	vector<int> a(n);
	for(auto &element : a)
		cin >> element;

	long long res = 0; int elCount = 0;
	unordered_set<int> S;
	for(int i = 0; i < n and elCount < k; i++) {
		if(!S.count(a[i])) {
			S.insert(a[i]);
			res += (i - elCount);
			elCount++;
		}
	}

	if(elCount == k)
		cout << res;
	else
		cout << -1;
	
	return 0;
}