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

Example Input: Sentence: 'two men in black wet suits beside a white surfboard'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'there is an old stove with old pots on the burners'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'doors of a silver train with people approaching it'. Find frequency of the letter 'e'
Example Output:
3