fences.py

lookup = {'N' : (1,0), 'S' : (-1,0), 'W' : (0,-1), 'E' : (0,1)}

def tuple_add(a, b):
    return (a[0] + b[0], a[1] + b[1])

def solve(data):
    pos = (0,0)
    solution = 0
    vertices = set([pos])
    fences = set()
    on_line = False

    for instruct in data.upper():
        new_pos = tuple_add(pos, lookup[instruct])
        edge = frozenset([pos, new_pos])
        
        if on_line is False and edge in fences:
            solution += 1
            on_line = True
        elif new_pos in vertices and edge not in fences:
            solution += 1
            on_line = True
        elif edge not in fences:
            on_line = False
        fences.add(edge)
        vertices.add(new_pos)
        pos = new_pos

    return solution

print(solve("wwweessnnesensensenweswensewnenwenewwwwwwwennnnsssssswe"))

# with open("gates.in", "r") as infile:
#     N = int(infile.readline())
#     data = infile.readline()
#     #assert len(data) == N

# with open("gates.out", "w") as outfile:
#     outfile.write(str(solution))