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

Example Input: Sentence: 'a traffic signal by a wire filled with some birds'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'the man is holding a plastic sled with snow in it next to a child'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'a green train engine sitting at the end of the train and next to the buidling'. Find frequency of the letter 'b'
Example Output:
1