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 knife a cutting board some bananas and a toaster'. Find frequency of the letter 'i'
Example Output: 2

Example Input: Sentence: 'a yellow traffic light sitting next to a street light'. Find frequency of the letter 't'
Example Output:
9