Solution

By dividing the contour into a set of small magnetic dipoles in the shape of a square, with the current in the touching sides directed in opposite directions, this combination, with the side of the square tending to zero, will give us a field equivalent to the field of the contour. At large field distances from such a sum of squares, there will be an equidistant dipole in the field of one dipole, and the number of dipoles in the contour will depend only on its area.