Exact Differential Equation

Form: $$M \, dx + N \, dy = 0$$ where $$\frac{\partial M}{\partial y} = \frac{\partial N}{\partial x}$$.
Solution: Find $$\psi(x,y)$$ such that $$\frac{\partial \psi}{\partial x} = M$$ and $$\frac{\partial \psi}{\partial y} = N$$.
General solution: $$\psi(x,y) = C$$.