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#include <iostream>
#include <vector>
#include <algorithm>
#include<string>
#include<fstream>

using namespace std;

const int maxA = 500000;
int n, k, tab[maxA], byl[maxA], ostByl = 0, ile[maxA], zaw[maxA];

bool zastap(int x, bool porz)
{
	int L = 0;
	int R = ostByl;
	while (L < R)
	{
		int sr = (L + R + 1) / 2;
		if (byl[sr] > x)
			R = sr - 1;
		else
			L = sr;
	}
	if (byl[L] == x && !porz)
		return true;

	if (L == 0 && x < byl[L])
		L = -1;

	for (int i = ostByl; i > L; i--)
		byl[i + 1] = byl[i];
	ostByl++;
	byl[L + 1] = x;
	return false;
}

bool spelnia()
{
	byl[0] = min(tab[0], tab[1]);
	byl[1] = max(tab[0], tab[1]);
	if (byl[1] == byl[0])
		return false;
	ostByl = 1;
	for (int i = 2; i < k; i++)
		if (zastap(tab[i], false))
			return false;
	return true;
}

int main()
{
	long long wynik = 0;
	cin >> n  >> k;
	for (int i = 0; i < n; i++)
	{
		cin >> tab[i];
		ile[tab[i]]++;
	}
	int ileRodz = 0, nadwyzka = 0, dobrze = 1;
	for (int i = 0; i < n; i++)
		if (ile[i] > 0)
			ileRodz++;
	if (ileRodz < k)
	{
		cout << -1;
		return 0;
	}
	if (spelnia())
	{
		cout << 0;
		return 0;
	}
	for (int i = 0; i < k; i++)
		byl[i] = 0;
	if (tab[0] == tab[1])
	{
		byl[0] = tab[0];
		nadwyzka = 1;
		ostByl = 0;
	}
	else
	{
		byl[0] = min(tab[0], tab[1]);
		byl[1] = max(tab[0], tab[1]);
		dobrze = 2;
		ostByl = 1;
	}
	for (int i = 2; i < n; i++)
		if (!binary_search(byl, byl + ostByl + 1, tab[i]))
		{
			zastap(tab[i], true);
			wynik += i - dobrze;
			dobrze++;
		}
	cout << wynik;
	return 0;
}