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

Example Input: Sentence: 'a tv in a glass mirror in the bathroom'. Find frequency of the letter 'h'
Example Output: 2

Example Input: Sentence: 'a yellow and green train on a train track in a wooded area'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'two people skateboarding down a sidewalk next to a street'. Find frequency of the letter 't'
Example Output:
6