  This paper derives an equality between the slab functions in Gross-Siebert
program and generating functions of open Gromov-Witten invariants for toric
Calabi-Yau manifolds, and thereby confirms a conjecture of Gross-Siebert on
symplectic enumerative meaning of slab functions. The proof is based on the
open mirror theorem of Chan-Cho-Lau-Tseng \cite{CCLT13}. It shows an instance
of correspondence between tropical and symplectic geometry in the open sector.
