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

Example Input: Sentence: 'a row of vintage cars in a grassy field'. Find frequency of the letter 'v'
Example Output: 1

Example Input: Sentence: 'there is no image here to provide a caption for'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'people in a skiing competition are going down a snow covered hill'. Find frequency of the letter 'g'
Example Output:
3