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

Example Input: Sentence: 'the little girl is flying her kite in the field'. Find frequency of the letter 'i'
Example Output: 7

Example Input: Sentence: 'a green bus driving in a street filled with heavy traffic'. Find frequency of the letter 'v'
Example Output: 2

Example Input: Sentence: 'a cat sleeping on a quilt on a bed'. Find frequency of the letter 'd'
Example Output:
1