It would be best to go to the width of the number field, so if you have
field1 N 5,0 and field 2 n 5,0
and a value in field1 of 1234 and a value in field2 of 456 you would have a key of 0123400456 if you lead padded or 1234045600 trailing . Doing this prevents addition cuasing duplicate values.
f1 = 1234 + f2 = 1234 = key = 2468 = f1 2000 + f2 = 468

In our case, f1 alwasy has 4 character (YYYY) but f2 has up to 5 didgits 1-99999
so a key would be 2000000123 or 1999000123