sum of textbox

Question

i have textbox1 and button1 

i enter textbox1 value is 12

when i click button textbox1 value same value shown in textbox2

 

then 

 after button click again

i enter in textbox3 value is 15

textbox4 value is  = 5

 

my question is i want sum(textbox5 ) =  textbox2 +  textbox3+textbox4

 

iam trying but error show attemted to divided by zero. 

 

plz help us