I need to make a proof but I can't come to the solution:
For every vertex of oriented graph with vertices $U_{1},U_{2},\ldots,U_{n}$ we've got $s_{+}(U)$ the number of edges, which come to the vertex $U$, and $s_{-}(U)$ the number of edges which leave from the vertex.
Prove that: $\sum_{i=1}^{n} |(s_{+}(U_{i})-s_{-}(U_{i})|$ is even number.
Until now I came to the statement that when we remove absolute values we get number 0.