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#include<bits/stdc++.h>
#define fi first
#define se second
using namespace std;
const int N=5e5;
int tab[N+10];
bool vis[N+10];
int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	cout.tie(NULL);
	int n,k;
	cin>>n>>k;
	long long ans=0,cnt=0;
	for(int i=1;i<=n;i++)
	{
		cin>>tab[i];
		if(k==0)
			continue;
		if(vis[tab[i]])
			cnt++;
		else
		{
			vis[tab[i]]=true;
			k--;
			ans+=cnt;
		}
	}
	cout<<(k==0 ? ans:-1)<<"\n";
	return 0;
}