To arrive the output

Question

i have one textbox field and checkbox field.in check box field i have enter up to 100 numbers.

what number i choose that number will be displayed in the textbox is correctly working.

but i want,for example  textbox  checkbox i choose 12 first and then i choose 14 but i want the ouptut in textbox as 1214 should be displayed.how to do.using asp.net with csharp.

please help me.