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

Example Input: Sentence: 'an elephant with broken tusks stands in front of a hill and boulders'. Find frequency of the letter 'n'
Example Output: 7

Example Input: Sentence: 'a baby elephant walks beside an adult elephant along a post and cable'. Find frequency of the letter 'l'
Example Output: 6

Example Input: Sentence: 'two pepperoni pizzas and a cup on a counter'. Find frequency of the letter 'u'
Example Output:
2