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

Input: Consider Input: Sentence: 'a gray elephant is in a grassy field'. Find frequency of the letter 'e'

Output: 3

Input: Consider Input: Sentence: 'a person dressed all in black snowboarding down a slope'. Find frequency of the letter 'e'

Output: 4

Input: Consider Input: Sentence: 'a bear sitting down on top of several rocks'. Find frequency of the letter 'r'
Output: 3