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

Example Input: Sentence: 'a girl laying on a bed posing for a picture'. Find frequency of the letter 'i'
Example Output: 4

Example Input: Sentence: 'a dog has found a rather large stick for playing fetch'. Find frequency of the letter 't'
Example Output: 3

Example Input: Sentence: 'a slightly burnt pizza is served on a platter'. Find frequency of the letter 'a'
Example Output:
4