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

Example Input: Sentence: 'a man with a large grey and black beard and glasses and a tie'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'several zebras  and a giraffe in an enclosure'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'a women is holding a baby next to a dog, a patio table, and a pool'. Find frequency of the letter 'e'
Example Output:
3