Peter M.Akhmet'ev A high-order analog of the helicity number for a pair of divergent-free vector field. {\bf Abstract:} Let $B, \tilde B$ be a pair of divergent free vector fields in $\R^3$ with a compact support. We constructs a high-order analog $M(B, \tilde B)$ of the Gauss helicity number (order 1) $H(B, \tilde B)=\int A \tilde B d\R^3$, $curl(A)=B$. The number $M$ is an invariant of volume preserved diffeomorphism with a compact support. It is presented by the following integral expression. For arbitrary 4 points $x_1,x_2,\tilde x_1, \tilde x_2$ we construct a polylinear function $m(x_1,x_2,\tilde x_1, \tilde x_2;B(x_1),B(x_2), \tilde B(\tilde x_1), \tilde B(\tilde x_2))$, invariant under a permutation of the points in each pair. The invariant $M$ is the mean value of m over arbitrary configuration of points. To clarify the geometrical meaning of the invariant we assume that the fields $B, \tilde B$ are concentrated in two disjoin tubes $\{L_1, L_2\}$ with the flows $F_1, F_2$. We assume that the linking number of the tubes equals zero. Under this assumption we prove $$ M(B)=\beta(L_1, L_2)F_1^2F_2^2, $$ where $\beta$ is the Sato-Levine invariant (the Vassiliev invariant of order 3 for isotopy class of 2-component links).