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

Example Input: Sentence: 'a man sitting on a motorcycle near two other men'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'two people in a snowy place near a wooden fence'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a royal blue jar in a travel terminal'. Find frequency of the letter 'v'
Example Output:
1