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

Example Input: Sentence: 'a man riding on the back of a white horse next to other horses'. Find frequency of the letter 'o'
Example Output: 6

Example Input: Sentence: 'a doll with huge eyes and black hair with blunt bangs holds a red umbrella'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a close up of a doughnut near a drink'. Find frequency of the letter 'e'
Example Output:
2