Two non closed paths homotopic relative to the endpoints have the same winding number?

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Let $\gamma:[0,1]\rightarrow {C}$ be a path. The winding number $\omega(\gamma)$ is defined as $\omega(\gamma)=\frac{\theta(1)-\theta(0)}{2\pi}$. I have read that two paths homotopic to the endpoints have the same winding number also for non closed curves. I would like to proof it but could not find one online. Do you have any suggestions?