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

Example Input: Sentence: 'a truck is parked in front of a store'. Find frequency of the letter 't'
Example Output: 3

Example Input: Sentence: 'a man standing in front of a statue near a metal pole'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'the time on the clock sculpture in the city shows 1:42pm'. Find frequency of the letter 'c'
Example Output:
4