about window form text box problem

Question

i have issue in textbox

 

i have two text box

suppose in second textbox i have value=100.12

now i want to remove only .12 from this value after enter in first textbox with (-) or (+) sign

if i used (-) sign then minus from second textbox value

if i used (+)sign then plus in second textbox value

please help me

 

demo   1. -12

            2. 100.12    =100

 

1. +.12

2. 100.12      =100.24

 

not compulsory that first textbox value start with decimal with sign

may be decimal or may be integer