  The natural bundle $\pi:E\to M$ of almost-complex structures is considered.
The action of the pseudogroup of all diffeomorphisms of $M$ on the total space
$E$ is investigated. A nontrivial 1-st order differential invariant of this
action is constructed. It is proved that the Nijenhuise tensor of an
almost-complex structures is equal to zero iff the constructed invariant for
this structure is zero.
