# Permutation for balls [duplicate]

Possible Duplicate:
number of combination in which no two red balls are adjacent.

We have $N$ slots. They have to be filled with balls (either green or red), one ball for each slot. Green balls can not be placed in consecutive slots. Find the number of different arrangements.

Can some one help?

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## marked as duplicate by Gerry Myerson, Leonid Kovalev, Chris Eagle, t.b., Asaf KaragilaAug 17 '12 at 15:14

It might have been useful, and not difficult, to find the number of possibilities for $N=1$, $N=2$, $N=3$, and perhaps $N=4$.