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

Example Input: Sentence: 'a dog has found a rather large stick for playing fetch'. Find frequency of the letter 't'
Example Output: 3

Example Input: Sentence: 'a stop sign near a rail road crossing sign'. Find frequency of the letter 'g'
Example Output: 3

Example Input: Sentence: 'a batter swings at a pitch during a baseball game'. Find frequency of the letter 'i'
Example Output:
3