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 railroad train pulling up to the train platform'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a view of a city at night time, with many cars below'. Find frequency of the letter 'o'
Example Output:
2