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: 'two zebras feed on a plant through some wire mesh'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'i'm sorry i have no idea what this is'. Find frequency of the letter 'h'
Example Output:
3