The figure shows two infinite semi-cylindrical shells: shell$-1$ and shell$-2$. Shell$-1$ carries current $i_1$, in inward direction normal to the plane of paper, while shell$-2$ carries same current $i_1$ , in opposite direction. A long straight conductor lying along the common axis of the shells is carrying current $i_2$ in direction same as that of current in shell$-1$. Force per unit length on the wire is