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

Example Input: Sentence: 'an old woman is holding her cat in front of a camera'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a passenger train that is going over a bridge'. Find frequency of the letter 't'
Example Output: 3

Example Input: Sentence: 'a girl holding a cellphone squatting down on the corner of a street'. Find frequency of the letter 'o'
Example Output:
6