( m , n ) can be reached either through ( m - 1 , n ) or through ( m , n - 1 )
return pathCountRec ( $ mat , $ m - 1 , $ n , $ k - $ mat [ $ m ] [ $ n ] ) + pathCountRec ( $ mat , $ m , $ n - 1 , $ k - $ mat [ $ m ] [ $ n ] ) ; }