Apply classic sudoku rules. Some intersections are marked by a number and an operator (+, -, x, /) in a circle. The number is the result of the operation, applied to both pairs of diagonally opposite cells. an "E" or "O" in the circle indicates that all four adjacent digits are even or odd respectively.
Notes:A question mark means that the result of the operation is not given and has to be determined as part of the puzzle. A question mark can replace any number (two-digit numbers or non-integer numbers are allowed too).