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

Example Input: Sentence: 'a person on a surfboard in the surf of a wave'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'a man in a black wet suit rides an orange surfboard on the water'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a man adjusting another mans suit in a building'. Find frequency of the letter 'n'
Example Output:
6

