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

Example Input: Sentence: 'a silver train crossing the road on tracks'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'a dog laying on the back of a boat in the water'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a half eaten doughnut in a persons hand'. Find frequency of the letter 'h'
Example Output:
3