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

Ex Input:
Sentence: 'two bears swim in a blue pool of water'. Find frequency of the letter 'o'

Ex Output:
4

Ex Input:
Sentence: 'man riding a skate board down the little board way'. Find frequency of the letter 'k'

Ex Output:
1

Ex Input:
Sentence: 'two astros fans use their cell phones in the stands'. Find frequency of the letter 'u'

Ex Output:
1