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

Ex Input:
Sentence: 'a stop sign near a rail road crossing sign'. Find frequency of the letter 'g'

Ex Output:
3

Ex Input:
Sentence: 'a black horse with a white dot on its forehead'. Find frequency of the letter 't'

Ex Output:
4

Ex Input:
Sentence: 'a woman holds the reins of a saddled horse'. Find frequency of the letter 'm'

Ex Output:
1