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

Example Input: Sentence: 'a large grass pasture land with sheep grazing on grass'. Find frequency of the letter 'a'
Example Output: 7

Example Input: Sentence: 'a long row of tables displaying lots of fruit'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a group of baseball players standing on a field'. Find frequency of the letter 'a'
Example Output:
6