Distinct strings

After encounter with dementors in the train Harry felt that his eyesight has degraded so he goes to Madam Poppy Pomfrey.Hogwarts matron.She makes a unique arrangement for his eye testing.She keeps an M×N grid containing lowercase alphabets at a distance and asks Harry to look into it and tell all the distinct strings visible using across reading order(i.e row wise from left to right).She tells tha