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

Example Input: Sentence: 'a woman is looking at sheep off in a field'. Find frequency of the letter 'm'
Example Output: 1

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: 'giraffes walking on dirt area next to fence in zoo setting'. Find frequency of the letter 's'
Example Output:
2