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

Example Input: Sentence: 'a woman in a messy kitchen with a child'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a man in a black wet suit rides an orange surfboard on the water'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'the two people are playing with their black dog'. Find frequency of the letter 'h'
Example Output:
3