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

Example Input: Sentence: 'a crowd of people skiing and enjoying the snow'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'three woman are having a discussion in the kitchen'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a glass of white wine with the bottle in the background'. Find frequency of the letter 'b'
Example Output:
2