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

Example Input: Sentence: 'a child sitting a table with a pizza in front of him'. Find frequency of the letter 'h'
Example Output: 3

Example Input: Sentence: 'soccer players on opposite teams fight for control over a ball'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a clean small kitchen area with white furniture'. Find frequency of the letter 'u'
Example Output:
2