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

Example Input: Sentence: 'a man riding a white surfboard on a large wave'. Find frequency of the letter 'a'
Example Output: 7

Example Input: Sentence: 'a blue fire hydrant painted on the bottom of it'. Find frequency of the letter 'h'
Example Output: 2

Example Input: Sentence: 'girl friends sharing the contents of their flip telephones'. Find frequency of the letter 'e'
Example Output:
7