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

Example Input: Sentence: 'a photo of a small boat on a shore'. Find frequency of the letter 'f'
Example Output: 1

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: 'a double deck tour bus being towed by a truck'. Find frequency of the letter 'o'
Example Output:
3