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

Example Input: Sentence: 'a herd of giraffes in tall african grass'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a toilet filled with a brown liner and dirty water'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a man rides a wave on his surfboard in a wetsuit'. Find frequency of the letter 'o'
Example Output:
2