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

Example Input: Sentence: 'a young girl is brushing her teeth in this dated photo'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a golden haired dog looking out of an open window'. Find frequency of the letter 'd'
Example Output: 4

Example Input: Sentence: 'a dog looks at the half eaten slice of pizza'. Find frequency of the letter 'l'
Example Output:
3