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

Example Input: Sentence: 'a young man with glasses on is taking a big bite'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'two bears swim in a blue pool of water'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'this giraffe is the grass he is an adult giraffe'. Find frequency of the letter 't'
Example Output:
3