“Do not worry about your difficulties in mathematics. I can assure you mine are still greater.”

Kafka writes it to Felice Bauer, and the joke is that the man who produced some of the most demanding prose of the twentieth century is consoling himself with the thought that his sums are worse. The line is the most self-deprecating sentence he wrote, and it is why it is still quoted by people who have never read a word of Kafka.