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

Example Input: Sentence: 'a bunch of green bananas is hanging on a tree'. Find frequency of the letter 'n'
Example Output: 7

Example Input: Sentence: 'a person on a skateboard going down a ramp'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'many bunches of bananas are hanging in front of a small store'. Find frequency of the letter 'h'
Example Output:
2