def calc(S):
    cnt = 0
    N = len(S)
    ans = 0
    for i in range(N):
        if S[i] == "o":
            cnt += 1
        else:
            ans = max(ans, cnt)
            cnt = 0
    return max(ans, cnt)

D = int(input())
C = []
C.extend(list(input()))
C.extend(list(input()))
N = len(C)
ans = 0
for i in range(N):
    temp = [None] * N
    for j in range(N):
        if i <= j < i + D:
            temp[j] = "o"
        else:
            temp[j] = C[j]
    ans = max(ans, calc(temp))
print(ans)