const int BOARD_DIM = 4;
const int BOARD_SIZE = BOARD_DIM * BOARD_DIM;
signed char adjacent[BOARD_SIZE][5] = {
  1,   4,   -1,  -1,  -1,
  0,   2,   5,   -1,  -1,
  1,   3,   6,   -1,  -1,
  2,   7,   -1,  -1,  -1,
  0,   5,   8,   -1,  -1,
  1,   4,   6,   9,   -1,
  2,   5,   7,   10,  -1,
  3,   6,   11,  -1,  -1,
  4,   9,   12,  -1,  -1,
  5,   8,   10,  13,  -1,
  6,   9,   11,  14,  -1,
  7,   10,  15,  -1,  -1,
  8,   13,  -1,  -1,  -1,
  9,   12,  14,  -1,  -1,
  10,  13,  15,  -1,  -1,
  11,  14,  -1,  -1,  -1,
};