a(n) = binomial(2*n-4,n-1)*(n+3)/n.

Terms

a(0) =4a(1) =0a(2) =2a(3) =7a(4) =24a(5) =84a(6) =300a(7) =1089a(8) =4004a(9) =14872a(10) =55692a(11) =209950a(12) =795872a(13) =3031032a(14) =11589240a(15) =44462565a(16) =171085500a(17) =660009840a(18) =2551955340a(19) =9887121090a(20) =38374857840a(21) =149184555000a(22) =580807904040