Adding one or sequential number in NUMC fields in custom table

Hi gurus,

I have the following requirement,

Budget cab be upload every month for next three month. Examp : apr for apt/may/jun and may for may/jun/jul and jun for jun/jul/aug like.

hence i created custom table with required field, also i created one NUMERIC field (data type - NUMC and data element - NUMER) getting sequential or next numer.

based on the month budget upload

first upload should be 1 and next upload should be 2 and next upload should be 3 ....

hence this numeric field can be increasing by 1 based on the descending order

SORT NUMERIC1 DESCENDING.

ZNUMERIC1 = ZNUMERIC1 + 1.

But i am getting error as "NUMC cannot converted as number".

Please help. How to get the next number based on the numeric field by select and sorting by descending order.