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#include<iostream>
#include<set>
using namespace std;

int main()
{
	int n, k, c = 0, t = 0, b;
	set<int> bb;
	cin >> n >> k;
	for (int i = 0; i < n; i++)
	{
		cin >> b;

		if (bb.count(b) == 0)
		{
			bb.insert(b);
			t += i - c;
			c++;
		}
	}
	cout << (c >= k ? t : -1);

	return 0;
}