Who's good at elastic potential energy problems?

Two lab carts have a light spring compressed between them and are held together by a string. The 1kg cart is on the left and the 2kg cart is on the right, on a flat surface. When the carts are at rest the string is cut and the 2 kg cart moves to the right with a speed of 1 m/s. If the total mechanical energy of the system is conserved, what was the elastic potential energy of the spring before the string was cut?