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

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 mixed bouquet of flowers in a glass vase set before a window'. Find frequency of the letter 'w'
Example Output: 3

Example Input: Sentence: 'a pair of giraffes looking opposite directions in a forest'. Find frequency of the letter 'f'
Example Output:
4