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

Example Input: Sentence: 'a man flying through the air on a bicycle'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a couple of boys in a field playing baseball'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'two children playing tee ball in the yard'. Find frequency of the letter 'd'
Example Output:
2