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

Example Input: Sentence: 'two bigger elephants standing with a small elephant'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a person does a trick on a ramp on a snowboard'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'a sky view of a city and the building in it'. Find frequency of the letter 's'
Example Output:
1