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

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: 'a black computer with a screen reading "press any key to begin."'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'chefs are preparing food in a busy kitchen'. Find frequency of the letter 'h'
Example Output:
2