23208

The reason that it is impossible to get any other number than 18 is because; the shortest path from start to finish is 2 left and 3 upwards, leaving you with 18 (10+1+1+2+2+2,). Therefore, any other move to attempt getting less points by subtracting (moving left/down) would be met by an equal addition of the same number (moving right/upward).
So ultimately any path (no matter how complex)would lead to 18.