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

Example Input: Sentence: 'a woman and a child walking down a street with a kite'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a woman walking on a shore next to a river'. Find frequency of the letter 'v'
Example Output: 1

Example Input: Sentence: 'a group of skiers posing for a photo'. Find frequency of the letter 'p'
Example Output:
3