Normal MLE I

Suppose that $X_1 \mid \mu, \dots, X_n \mid \mu \sim N(\mu,\sigma^2)$ IID, where $\sigma^2 > 0$ is known. If it exists, find the MLE of $\mu$ with the data $\{1,-2,4,-1\}$ and $\sigma^2 = 1$. If it doesn't exist, report $-1$.