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

Example Input: Sentence: 'a baseball player dropping his bat and starting to run'. Find frequency of the letter 'o'
Example Output: 2

Example Input: Sentence: 'pizza with different vegetables sitting on top of a wooden slab'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a baseball player in a blue shirt throwing a ball'. Find frequency of the letter 'e'
Example Output:
3