not ip address

["Mihamina Rakotomandimby (R12y)" <mihamina@lab.vectoris.fr>]

Hi all
I would like to write this sentence in iptables rule:
  "REJECT everything having port #443 (httpS) as destination except
   if the IP address that query it are one of 192.168.0.18 or 192.168.0.50"

I'd like to forbid httpS/443 traffic except for those two IP addresses.

I made:

$IPT -t nat -A PREROUTING  ! --source 192.168.0.50  \
      -p tcp --dport 443 -j REJECT

and

$IPT -t nat -A PREROUTING  --source ! 192.168.0.50  \
      -p tcp --dport 443 -j REJECT

But they are all bad syntax.

Would you please help me to find my mistake?
I'm running iptables v1.4.0

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