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#include <bits/stdc++.h>
using namespace std;
int n,k,y;
long long ans;
int a[500005],b[500005],c[500005];
queue<int>q;
bool u;

int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cin>>n>>k;
for(int i=0;i<n;i++)
{
	cin>>a[i];
	if(b[a[i]] == 0)
		q.push(i);
	b[a[i]] ++;
}
u = true;
for(int i=0;i<k;i++)
{
	if(c[a[i]] != 0)
	{
		y = 0;
		while(q.empty() == 0 && (y <= i || c[a[y]] > 0))
		{
			y = q.front();
			q.pop();
		}
		if(y <= i || c[a[y]] > 0)
			u = false;
		else
		{
			ans += (y - i);	
			c[a[y]] ++;
		}
	}
	else
		c[a[i]] ++;
}
if(u)
	cout<<ans;
else
	cout<<-1;
	
return 0;	
}