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

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

Example Input: Sentence: 'a woman in yellow dress standing next to a horse'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'a school bus parked next to a curb and sidewalk'. Find frequency of the letter 'u'
Example Output:
2