In this task, you need to count the number of times the given letter appears in the given sentence.

Example Input: Sentence: 'a scene at night; a plane alone on a runway with lit-up city buildings on the horizon'. Find frequency of the letter 'w'
Example Output: 2

Example Input: Sentence: 'the man is holding a plastic sled with snow in it next to a child'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'a transit train with several cars is viewed in the distance'. Find frequency of the letter 'e'
Example Output:
6