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

Example Input: Sentence: 'a small boat travels on the water next to the shore'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'two indian women in colorful garb standing barefoot with two indian men'. Find frequency of the letter 'd'
Example Output: 3

Example Input: Sentence: 'a person riding a surf board while holding a parachute'. Find frequency of the letter 'g'
Example Output:
2