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

Example Input: Sentence: 'a black horse with a white dot on its forehead'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a bowl of granola, dried fruit, and bananas'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a man laying on a surfboard surfing in the surf'. Find frequency of the letter 'o'
Example Output:
2