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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;
	
	set<int> had;
	long long result = 0;
	int last_set = 0;
	
	for (int i = 0; i < n; i++) { 
		int a; cin >> a;
		
		if (had.count(a) == 0) {
			had.insert(a);
			result += i - last_set;
			last_set++;
		} 
		
		if (last_set == k) 
			break;
	}
	
	if(last_set < k) result = -1;
	
	cout << result << endl;
	
	return 0;
}