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

Example Input: Sentence: 'a line of people on motorcycles on the road'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a woman is looking at sheep off in a field'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'a modern looking simple bathroom with wood floors'. Find frequency of the letter 'l'
Example Output:
3