MLE for a log-normal on log-scale

Daily traded volumes are modeled as log-normal, meaning $\ln X_i$ is normal with mean $\mu$ and variance $\sigma^2$. The log-normal density is

$f(x \mid \mu, \sigma^2) = \frac{1}{x\sqrt{2\pi\sigma^2}}\, \exp\!\left(-\frac{(\ln x - \mu)^2}{2\sigma^2}\right), \qquad x > 0.$

Taking natural logs of five observations gives $y_i = \ln x_i$ equal to 2.0, 2.5, 3.0, 3.5, 4.0.

Derive the maximum likelihood estimator of $\mu$ and evaluate it.