textbox validation

Question

hello there 

i have just create a software in which 6 textbox is there the textbox 1and 2 is for input and textbox 3 is for output which is the addition of the texbox 1 and 2. 

and again textbox 4and 5 is for input and textbox 6 is for output which is addition of textbox 4 and 5.

the problem is that when i use only textbox 1,2and 3 it gives error pls any body help me out how can i ignore the textbox 4,5and 6 .