Shown below is a number puzzle (4×4 sliding puzzle; rows: 10 8 3 14 / 7 5 12 2 / 4 9 x 11 / 13 1 15 6). Only one piece can be slided horizontally or vertically in one move. What is the least number of moves required to bring the piece with number 10 to the empty position x? [Figure: see page 3 of source PDF]