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

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 man and a woman are walking down the street with an umbrella over their heads'. Find frequency of the letter 'w'
Example Output: 4

Example Input: Sentence: 'a large building with a clock stands out against the sky'. Find frequency of the letter 'i'
Example Output:
4