D = input()
C = [False] * 7
for i in range(2):
    week = raw_input()
    for day in week:
        if day == 'o':
            C.append(True)
        else:
            C.append(False)
C.extend([False] * 7)


def ok(i, j):
    return C[i:(i+j)].count(False) <= D

ans = 0
for i in range(len(C)+1):
    for j in range(1, len(C)-i+1):
        if ok(i, j):
            ans = max(ans, j)
print(ans)