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

Example Input: Sentence: 'a river with a long dragon boat in it and rowers on the boat in sets of two, along with two other people'. Find frequency of the letter 'h'
Example Output: 4

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: 'someone's toes that are not covered by a blanket'. Find frequency of the letter 't'
Example Output:
5

