15

Fifteen is the magic constant of the unique order-3 normal magic square:

In recreational mathematics, a magic square of order n is an arrangement of n-square numbers, usually distinct integers, in a square, such that the n numbers in all rows, all columns, and both diagonals sum to the same constant. A normal magic square contains the integers from 1 to n-square.

Magic square is well known in Japan as the puzzle "Sudoku".