  We prove a Littlewood-Richardson type formula for
$(s_{\lambda/\mu},s_{\nu/\kappa})_{t^k,t}$, the pairing of two skew Schur
functions in the MacDonald inner product at $q = t^k$ for positive integers
$k$. This pairing counts graded decomposition numbers in the representation
theory of wreath products of the algebra $C[x]/x^k$ and symmetric groups.
