The digit in the square cell is 2, 4, 6, or 8. The rest of the clues in the grid are “double arrow” clues ie The sum of the digits along a line is equal to the sum of the digits in the circles at either end of the line. Additionally, the grid contains 9 “doublers”, one in each row column and box. For purposes of double arrow clues, cells on doublers count as double their value. Each digit from 1 through 9 is doubled exactly once.