Textbox sum

Question

im kinda little noob in c# so i need you help.

hello guys im having a problem on my textbox, i have a textbox  and it is multi line, what i want is sum all the numbers on my textbox 
and display it to txtboxSum
for example,

textbox

4 
5
2
1
4


sum

but i dont know how to do this, i dont have any idea. i hope that you can help me guys thanks