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

Example Input: Sentence: 'a downhill skier with snow plumes behind them'. Find frequency of the letter 'i'
Example Output: 4

Example Input: Sentence: 'four women around a man sitting in front of a cake'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a red and white stop sign and a street sign'. Find frequency of the letter 't'
Example Output:
4