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

Example Input: Sentence: 'two persons standing outside a moving sub train'. Find frequency of the letter 'a'
Example Output: 3

Example Input: Sentence: 'a group of people stand inside of a big room'. Find frequency of the letter 'o'
Example Output: 6

Example Input: Sentence: 'a red train moving under a series of blue arches'. Find frequency of the letter 'd'
Example Output:
2