A cube-shaped container has vertices A, B, C, and D, where AB and CD are parallel edges of the cube, and AC and BD are diagonals of faces of the cube, as shown. Vertex A of the cube is set on a horizontal plane P so that the plane of the rectangle ABDC is perpendicular to P, vertex B is 2 meters above P, vertex C is 8 meters above P, and vertex D is 10 meters above P. The cube contains water whose surface is parallel to P at a height of 7 meters above P. The volume of water is nm cubic meters, where m and n are relatively prime positive integers. Find m+n.