74 is a sad number. In number theory, a happy number is a number which eventually reaches 1 when the number is replaced by the sum of the square of each digit. For instance, 13 is a happy number because 1^2 + 3^2 = 10, and 1^2 + 0^2 = 1. On the other hand, 4 is sad number because the sequence starting with 4^2 = 16 and 1^2 + 6^2 = 37 eventually reaches 2^2 + 0^2 = 4, the number that started the sequence, and so the process continues in an infinite cycle without ever reaching 1.