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

Example Input: Sentence: 'people waiting on a train platform in a wooded area'. Find frequency of the letter 'p'
Example Output: 3

Example Input: Sentence: 'two men in black wet suits beside a white surfboard'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'ski boarder getting up from fall in the snow'. Find frequency of the letter 'l'
Example Output:
2