Bioclimatic variables - wettest quarter

Hello All,

I am attempting to reproduce the bioclimatic (Worldclim, Hijmans et al
2005) variables with the raster package and I am stuck on the group
"temp of driest quarter" etc. variables.
I am attempting it with the function "overlay" ("raster" package).

Here is how it looks:
# To know the first month of the wettest quarter
maxmonth<- function(a,b,d,e,f,g,i,j,k,l,m,n){
                     o<- sum(c(a,b,d))
                     p<- sum(c(b,d,e))
                     r<- sum(c(d,e,f))
                     u<- sum(c(e,f,g))
                     v<- sum(c(f,g,i))
                     w<- sum(c(g,i,j))
                     x<- sum(c(i,j,k))
                     y<- sum(c(j,k,l))
                     z<- sum(c(k,l,m))
                     aa<- sum(c(l,m,n))
                     ab<- sum(c(m,n,a))
                     ac<- sum(c(n,a,b))
                     ad<- which.max(c(o,p,r,u,v,w,x,y,z,aa,ab,ac))
                     return(ad)}