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

Ex Input:
Sentence: 'a plane with stairs next to it sitting in a large lot'. Find frequency of the letter 'i'

Ex Output:
6

Ex Input:
Sentence: 'a dog has found a rather large stick for playing fetch'. Find frequency of the letter 't'

Ex Output:
3

Ex Input:
Sentence: 'a man surfing on a wave in the ocean'. Find frequency of the letter 'g'

Ex Output:
1