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

Example Input: Sentence: 'group of snowboarders on a snowy trail overlooking a mountain'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'a glass vase sits between two statues'. Find frequency of the letter 'o'
Example Output: 1

Example Input: Sentence: 'a man that is standing on a surfboard in the water'. Find frequency of the letter 'i'
Example Output:
3