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

Example Input: Sentence: 'a man in a black wet suit rides an orange surfboard on the water'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'children with scissors and construction paper in a school classroom'. Find frequency of the letter 'c'
Example Output: 6

Example Input: Sentence: 'a traffic light with all green lights over a street'. Find frequency of the letter 'h'
Example Output:
3