Crystal Reports Group and sum of 2 columns

Message

     Hi, 

I need to group one column based on a condition (specific value) and then summarise the value of another column corresponding to this condition.

Example:

Column 1             Column 2
898                       hours
125.79                  units
3.00                      units
3.00                      hours

I want to group on column 2 and sum on column 1 to get:

Column 1             Column 2
901                       hours
128.79                  units

I have tried to enter the following code but get an error saying 'there must be a group that matched this field'

Sum ({@hrs & units joined},{@hrs or unit label} )

where 
column 1 = {@hrs & units joined} and 
column 2 ={@hrs or unit label}

i would appreciate any assistance

Regards, 
Despina