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

Example Input: Sentence: 'a blue fire hydrant painted on the bottom of it'. Find frequency of the letter 'h'
Example Output: 2

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 view of a large bridge in the foreground of a building with many spires'. Find frequency of the letter 'd'
Example Output:
3