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: 'a social occasion of people are uncovered before the open day'. Find frequency of the letter 'c'

Ex Output:
4

Ex Input:
Sentence: 'a tray on a table with food on the tray'. Find frequency of the letter 'r'

Ex Output:
2