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

Example Input: Sentence: 'a picture of two men on a motorcycle in front of a railroad track'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a black cat laying down on a bed'. Find frequency of the letter 'y'
Example Output: 1

Example Input: Sentence: 'two teams standing in a feild playing a soccer game'. Find frequency of the letter 'c'
Example Output:
2