SUM two different fields

I have two variable 

$V{SUM_ITEMS_SHIPPED} and $V{SUBREP_COUNT}

I want sum of these two variables using Groovy. For this, I dragged a new text field to summary band and in the "Text Field Expression" i have given as 

$V{SUM_ITEMS_SHIPPED} +$V{SUBREP_COUNT}

But the result of this concatenation of these two values and not the sum.

For ex: $V{SUM_ITEMS_SHIPPED} = 1

             $V{SUBREP_COUNT} = 0

output is "10" and not "1"

Please help me how to do this.

What "Text Field Expression" is needed to sum these two variables together ?