The 26 letters of the alphabet were divided into two piles according to a simple rule. Two letters were then swapped leaving the piles as follows.

Pile 1: a,c,e,g,j,o,q,s,u,v,w,y,z
Pile 2: b,d,f,h,i,k,l,m,n,p,r,t,x

Which two letters should be swapped to return each pile to its original composition?