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 men standing around two pizzas in a conference room'. Find frequency of the letter 't'
Example Output: 2

Example Input: Sentence: 'a body of water with boats next to a factory area'. Find frequency of the letter 'd'
Example Output:
1