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

Ex Input:
Sentence: 'a downhill skier with snow plumes behind them'. Find frequency of the letter 'i'

Ex Output:
4

Ex Input:
Sentence: 'an old glass vase containing flowers on a glass table'. Find frequency of the letter 'b'

Ex Output:
1

Ex Input:
Sentence: 'an open black suitcase and a persons arm'. Find frequency of the letter 'k'

Ex Output:
1