Find the number of injective functions (from finite set to finite set):
$$ f:\{ 1,2,3,4,5,6,7\} \rightarrow \{ 1,2,3,4,5,6,7,8,9\} $$
with the following property: $$ f(i) \neq f(j)+1\ \text{ for all } \ 1\leq i < j \leq 7. $$
Help me with mathematical decision (I can count this number on computer). Thank you.