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

Example Input: Sentence: 'a crowd of people standing in front of a green train'. Find frequency of the letter 'g'
Example Output: 2

Example Input: Sentence: 'a display case filled with lots of different clocks'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'two parrots sitting side by side on a perch'. Find frequency of the letter 'h'
Example Output:
1