The goal is to fill the square grid with numbers so that each number appears exactly once in each row and column. Some numbers may already be given at the start.
At the intersections of the grid lines, there may be circles with additional conditions. There are the following types of conditions:
Arithmetic value and operation sign (addition, subtraction, multiplication, division): the number in the circle indicates the result of applying the specified arithmetic operation to the pair of diagonally adjacent cells to the circle.
The letter "E" (for even): all four cells surrounding the circle must contain even numbers.
The letter "O" (for odd): all four cells around the circle must contain odd numbers.
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