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

Example Input: Sentence: 'man is snow gear and skis skiing in the snow'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'a river with a long dragon boat in it and rowers on the boat in sets of two, along with two other people'. Find frequency of the letter 'h'
Example Output: 4

Example Input: Sentence: 'the little boy is running with a baseball bat'. Find frequency of the letter 'n'
Example Output:
3