IMO Team Selection Contest I

Find the smallest real constant $C$ such that for any positive real numbers $a_1,a_2,a_3,a_4$ and $a_5$ (not necessarily distinct), one can always choose distinct subscripts $i,j,k$ and $l$ such that $\left|\frac{a_i}{a_j}-\frac{a_k}{a_l}\right|\le C$.