Testing — size of a test based on a sum of uniforms

Let $X_1$ and $X_2$ be a random sample from Uniform$[0,\theta]$ distribution, where $\theta>0$. For testing the hypothesis $H_0:\theta=1$ against $H_1:\theta=2$, consider a test which rejects $H_0$ if $X_1+X_2>\tfrac45$. Then, the probability of type-I error is