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

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

	long long n, k;
	cin >> n >> k;

	int marki = 0;
	vector <bool> uzyte(500007, false);
	long long suma = 0;

	int x, pomk = k;
	for(int i = 0; i<n && pomk; ++i){
		cin >> x;
		if(uzyte[x] == false){
			suma += i;
			uzyte[x] = true;
			pomk--;
			marki++;
		}
	}

	if(k > marki){
		cout << -1 << "\n";
		return 0;
	}

	long long odp = suma - (k-1) - (((k-1)*(k-2))/2);
	cout << odp << "\n";
}