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

Example Input: Sentence: 'a white and blue plate topped with meat and a salad'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a train station with a train pulling into the platform'. Find frequency of the letter 'i'
Example Output: 6

Example Input: Sentence: 'there is a bird perched on the rock by the beach'. Find frequency of the letter 'i'
Example Output:
2