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

Example Input: Sentence: 'four women around a man sitting in front of a cake'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a gray elephant is in a grassy field'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'two zebras standing close together in a zoo like environment'. Find frequency of the letter 'z'
Example Output:
2