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

Example Input: Sentence: 'some people sitting on surfboards in the water'. Find frequency of the letter 't'
Example Output: 4

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 floral plate with greens, carrots and potatoes'. Find frequency of the letter 'a'
Example Output:
6