RMO 2015 M1

Let $ABCD$ be a convex quadrilateral with $AB=a$ , $BC=b$ , $CD=c$ and $DA=d$ . Suppose $$a^2+b^2+c^2+d^2=ab+bc+cd+da,$$ and the area of $ABCD$ is $60$ sq. units. If the length of one of the diagonals is $30$ units, determine the length of the other diagonal.