The meaning of problems
There $ A $ a $ 0 $, $ b $ a $ 1 $, $ c $ a $ 2 $, $ d $ a $ 3 $, find how many of length $ n-$ does not contain $ 0123 $ this substring of string a number.
$ N \ the 1000 \ space a + b + c + d \ $ 500
answer
Push convolution equation 1
The method of inclusion and exclusion 2
Made it clear that no further explanation