Nonlinear Systems & Linearization · Easy problem

For the Lotka–Volterra system $x' = x(1 - y),\; y' = y(x - 1)$ with $x, y > 0$, the function $V(x, y) = x - \ln x + y - \ln y$ is constant along every trajectory. What is the value of $V$ on the trajectory through the point $(e, 1)$?