trigger msgbox when certain conditions are met

[ilovejazzmusic]

hello there
i've spent a long time looking for what i need on various forums. now it's time to post the specific question i'm trying to solve. i don't imagine that it will be too complicated for true experts (unlike me).

i want a message box to appear (with text that i shall write. something like "congratulations!") when 2 conditions are met.
these conditions are pretty simple.
one of them is if the sum of cells a1:a4 = 12 and the other is if the value in b1 = 1.
i guess these conditions need to trigger a macro which causes the message box to appear.
please help!
nik