Natural logarithm (ln)
The logarithm with base e ≈ 2.718: ln y is the power you raise e to in order to get y.
The standard answer is surprise = −ln p, where p is the probability the model gave the outcome and ln is the natural logarithm (log base e).
See it in action
Try natural logarithm (ln) hands-on in the lesson Softmax and cross-entropy, part of the The maths behind AI course.