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

Ex Input:
Sentence: 'two giraffes standing next to some trees'. Find frequency of the letter 'f'

Ex Output:
2

Ex Input:
Sentence: 'a group of people seated in chairs together and facing the same direction, with one man holding a soccer ball in his hands'. Find frequency of the letter 'm'

Ex Output:
2

Ex Input:
Sentence: 'an old woman is holding her cat in front of a camera'. Find frequency of the letter 'r'

Ex Output:
3