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 mixed bouquet of flowers in a glass vase set before a window'. Find frequency of the letter 'w'
Example Output: 3

Example Input: Sentence: 'the sony remote is aimed at the television'. Find frequency of the letter 'a'
Example Output:
2