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

Example Input: Sentence: 'two men with colorful ties are posing for a picture'. Find frequency of the letter 'r'
Example Output: 4

Example Input: Sentence: 'an elephant with broken tusks stands in front of a hill and boulders'. Find frequency of the letter 'n'
Example Output: 7

Example Input: Sentence: 'a softball player holding the bat on first base'. Find frequency of the letter 't'
Example Output:
4