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#include <algorithm>
#include <iostream>
#include <vector>

using namespace std;

int main()
{
	int n, k, a;

	cin >> n >> k;

	vector<int> seen(n + 1);
	int res = 0;
	int pos = 0;

	for (int i = 0; i < n; ++i)
	{
		cin >> a;

		if (!seen[a])
		{
			res += i - pos;
			++pos;
		}

		seen[a] = 1;

		if (pos == k)
			break;
	}

	if (pos != k)
		res = -1;

	cout << res << endl;

	return 0;
}