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

Example Input: Sentence: 'a man hitting a tennis ball with a tennis racquet'. Find frequency of the letter 't'
Example Output: 6

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: 'the front of train makes its way along the tracks'. Find frequency of the letter 'k'
Example Output:
2