Excel 2007 : Number format for millions and billions

I want a number format so that I can produce the display on the right when entering the number on the left:


12345 -> 12.35K
12345678 -> 12.35M
1234567890 -> 1.23B


Is this possible using the standard custom number formatting system?

I'm willing to give up the rounding and accept truncation if it makes it easier.