Mathrax

Mathrax is a Sudoku-like logic puzzle. Enter a number in each empty cell so that the diagonally adjacent cells satisfy a mathematical condition.

Rules: In each cell of a grid of size NxN, a number from the range 1-N is to be entered. In each row and column, each number must appear exactly once.
Some intersections of the grid lines are marked with circles in which a number and an operator (+, -, *, ÷) are indicated.
The number is the result of the operation when the operator is applied to the two diagonally opposite pairs of numbers. For an “e” in the circle, all four adjacent numbers must be even; for an “o” they must be odd.