On a board the following six vectors are written: (1,0,0),(−1,0,0),(0,1,0),(0,−1,0),(0,0,1),(0,0,−1). Given two vectors v and w on the board, a move consists of erasing v and w and replacing them with 21(v+w) and 21(v−w). After some number of moves, the sum of the six vectors on the board is u. Find, with proof, the maximum possible length of u.