translating ips and ports

I need to do the following translations:
dst: ip1/udp-port1 to dst:ip3/udp-port1
dst: ip2/udp-port2 to dst:ip3/udp-port2

then, in the reverse direction:
dst: ip3/udp-port1 to dst:ip1/udp-port1
dst: ip3/udp-port2 to dst:ip2/udp-port2

Any ideas?

http://www.mikrotik.com/documentation/manual_2.4/IP/NAT.html