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

Example Input: Sentence: 'a girl sits on a surfboard and smiles'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'two people and two children walking in a building'. Find frequency of the letter 'd'
Example Output: 3

Example Input: Sentence: 'a double decker bus waits at a bus stop'. Find frequency of the letter 't'
Example Output:
3