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

Ex Input:
Sentence: 'the man is holding a plastic sled with snow in it next to a child'. Find frequency of the letter 'g'

Ex Output:
1

Ex Input:
Sentence: 'a microwave oven with a yellow note on the door'. Find frequency of the letter 'w'

Ex Output:
3

Ex Input:
Sentence: 'red buses sitting in a line in front of a city building'. Find frequency of the letter 'n'

Ex Output:
6