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

Example Input: Sentence: 'some sheep are standing in a grassy field'. Find frequency of the letter 'n'
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 beautiful woman laying on top of a bed in lingerie'. Find frequency of the letter 'e'
Example Output:
4