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

Example Input: Sentence: 'there is an old stove with old pots on the burners'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a double decker bus with a lot of people on board'. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'frisbee disc athletes from one team are all smiles'. Find frequency of the letter 's'
Example Output:
5