Xorg/Twm window has focus only when mouse is over

I have Xorg with Twm and xterm installed on Arch linux, and when I start the X server, I can type in xterm only when the mouse pointer is over its window. I have been searching options to Xorg and Twm which can have something related with focus when mouse is over window but I didn't succeed. How can I set my system up so that focus isn't bound to the mouse position?