Advanced rollover buttons howto?

I need code for a movieclip (20 frames) controlled by a button and i used the following code (see below)
it doesn’t work… the else function doesn’t work. Can anybody help me out please??

on (rollOver) {
with(_root.but)
if (_currentframe = 1){
gotoAndPlay(2);
}
else {
gotoAndPlay(19-_currentframe);

}
}

on (rollOut) {
with(_root.but)
if (_currentframe = 10){
gotoAndPlay(11);
}

else {
gotoAndPlay(19-_currentframe);

}
}