Magnetic Field Due to a Straight Current Carrying Wire

As shown in the figure, a single conducting wire is bent to form a loop in the form of a circle of radius $r$ concentrically inside a square of side $a,$ where $a:r=8:\pi$. A battery $B$ drives a current $I$ through the wire. If the battery $B$ and the gap $G$ are of negligible sizes, determine the strength of magnetic field at the common centre $O$.