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

Example Input: Sentence: 'a man and several white red and blue airplanes'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a small boat travels on the water next to the shore'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'this appears to be a workplace celebration, possibly a birthday'. Find frequency of the letter 't'
Example Output:
4