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

Example Input: Sentence: 'man in red shirt with black suitcase standing outside of building'. Find frequency of the letter 'i'
Example Output: 8

Example Input: Sentence: 'a man sitting on a motorcycle near two other men'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'two people standing at a gate with a group of cars'. Find frequency of the letter 'o'
Example Output:
4