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

Example Input: Sentence: 'a big bear walks through some grass'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a pizza pie sitting on a wooden table with tongs'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a black and white photo of small old planes on a runway'. Find frequency of the letter 'o'
Example Output:
5