Allow SSH on port 443

[wary]

Hello!
Because from university I can only use port 443, I want to use SSH to my router on this port!
I don't want to change the port the SSH-server listens, because I want to use it from the internal network.
It works if I add the following IPTABLES rules

iptables -D INPUT -j DROP
iptables -A INPUT -p tcp --dport 443 -j ACCEPT
iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 443 -j DNAT --to-destination $4:22

But then, all ports are open
If I add

iptables -A INPUT -j DROP

it doesn't work anymore.
What do I need to change?