import itertools

d = int(input())
a = input()
b = input()
c = a + b

g = [[k, list(g)] for k, g in itertools.groupby(c)]

ans = 0

for i in range(len(g)):
    if g[i][0] == "x" and len(g[i][1]) == d:
        t = len(g[i-1][1]) + d + len(g[i+1][1])
        ans = max(ans, t)
print(ans)