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#include<bits/stdc++.h>
#define fi first
#define se second
#define pb push_back
using namespace std;

int tab[500009], col[500009];

int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	int n, k, a = 1, wyn = 0;
	cin>>n>>k;
	for(int i=1; i<=n; i++) cin>>tab[i];
	for(int i=1; i<=n; i++)
	{
		if(col[tab[i]] == 0)
		{
			col[tab[i]] = 1;
			wyn += i - a;
			a++;
			if(a == k+1) break;
		}
	}
	if(a < k+1) cout<<-1;
	else cout<<wyn;
	return 0;
}