Creating a 'Checksum'

Hi,

My Google skills have failed me.

I need a way to add together the numbers held in 2 fields to generate a simple checksum

e.g.

Field 1 = 12345

Field 2 = 23456

Checksum = 1+2+3+4+5+2+3+4+5+6 = 35

Hopefully a simple answer...

Many thanks

Andrew