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

Example Input: Sentence: 'a yellow and green train on a train track in a wooded area'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'a glass a rope and a pair of scissors'. Find frequency of the letter 's'
Example Output: 6

Example Input: Sentence: 'man holding a baby girl while staring at a frisbee'. Find frequency of the letter 'i'
Example Output:
5