2048 (Easy)

2048 is a single player puzzle game played on a $4 \times 4$ board.

One move pushes every block on the board in one of the four directions: up, down, left, or right. When two blocks of equal value collide, they merge into a single block whose value is twice as large. Within one move, a block that has already merged does not merge again during that same move. The original game adds a new block after every move, but in this problem no new block ever appears.

Figure 1	Figure 2	Figure 3

Moving the blocks up from Figure 1 gives Figure 2. Moving them left from there gives Figure 3.

Figure 4	Figure 5	Figure 6	Figure 7

Moving the blocks right from Figure 4 gives Figure 5, and moving them up from there gives Figure 6. Moving them right from Figure 6 gives Figure 7.

Figure 8	Figure 9

Moving the blocks left from Figure 8 makes the two 2 blocks collide and merge into a 4, which gives Figure 9.

Figure 10	Figure 11	Figure 12	Figure 13

Moving the blocks up from Figure 10 gives Figure 11. Moving them up from Figure 12 gives Figure 13, because a block that has already merged during a move does not merge again in that move.

Figure 14	Figure 15

When three blocks of equal value sit in a row, the two blocks nearer the direction of the move merge first. For a move upward, the upper two blocks merge. Moving up from Figure 14 gives Figure 15.

The 2048 game in this problem is played on a board of size $N \times N$. Given the board size and the initial arrangement of the blocks, write a program that finds the largest block value obtainable with at most 5 moves.

The first line contains the board size $N$. ($1 \le N \le 20$)

Each of the next $N$ lines contains $N$ values describing the initial board. A 0 is an empty cell and any other value is a block. The number written on a block is a power of two between 2 and 1024, inclusive. At least one block is present.

Print the largest block value obtainable with at most 5 moves. Making no move at all is allowed, so the answer is never smaller than the largest block on the initial board.