#include <iostream>
#include <vector>

using namespace std;

int main()
{
	int n=3, k=2;
	cin >> n >> k;
	vector<int>a;
	for (int i = 0; i < n; i++)
	{
		int m;
		cin >> m;
		a.push_back(m);
	}
	vector<int>byly;
	int t = 0;
	byly.push_back(a[0]);
	for (int i = 1, x = 1; x < k && i < n; i++)
	{
		int j = 0;
		while (j < byly.size() && a[i] != byly[j]) j++;
		if (j < byly.size()) continue;
		byly.push_back(a[i]);
		t += i-x;
		x++;
	}
	if (byly.size() == k) cout << t;
	else cout << -1;
}