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

Example Input: Sentence: 'town clocks affixed to a tall, brick building'. Find frequency of the letter 'r'
Example Output: 1

Example Input: Sentence: 'two elephants facing each other touching trunks in an enclosure'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'the little girl is flying her kite in the field'. Find frequency of the letter 'i'
Example Output:
7