## Haskell, <s>49</s> 43 bytes a n|n<4=1|0<1=sum[a k*a(n-2-k)|k<-[2..n-1]] [Try it online!](https://tio.run/##BcHBCoAgDADQX9mhQwUbKkEX/RLxsEOQqEOybv77eu/mUa5aVRlkij@CncbbML4WGcrOq6DDss3iMToiQZuSNs4CAfqT5YUFGndgiIbInkl/ "Haskell – Try It Online") Edit: -6 bytes thanks to @Jo King.