Normal MLE II

Suppose that $X_1 \mid \theta, \dots, X_n \mid \theta \sim N(\mu,\sigma^2)$ IID, where $\mu$ is known but $\sigma^{2}$ is unknown. If it exists, find the MLE of $\sigma^2$ with the data $\{1,-2,4,-1\}$ and $\mu = 1$. If it doesn't exist, report $-1$.