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 taking a sip of a drink from a glass'. Find frequency of the letter 'i'
Example Output: 4

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 bathroom that has some tiles missing and items in the tub'. Find frequency of the letter 'i'
Example Output:
5