D = int(input())
C = input()
C = C + input()
N = [0]
for i, c in enumerate(C, start=1):
    if c == "x":
        N.append(i)
N += list(range(15,30))
ans = 0
for i in range(len(N) - D - 1):
    ans = max(ans, N[i + D + 1] - N[i] - 1)
print(ans)