One key word that is not mentioned clearly in the problem is that we can NOT place a "-" to the cell IF the cell is not "-" currently.

This can make many programmes WA.

For example, for n = 1, this code is NOT acceptable:
3
1 - 2 + >
2 # 2 # <
2 + 2 - =  (Here we can NOT replace a "+" with a "-")

while this one is acceptable:
4
1 - 2 - >
2 - 3 - <
2 # 4 # <
3 - 1 + >

Hope the ADMIN to make it clear in the STATEMENT.
This problem has driven me crazy for months!!!

Good luck.