D = int(input())
C = input() + input()
def goldenWeek(day):
    return 14 - ['o' * i in day for i in range(15)][::-1].index(True)

ans = 0
for i in range(14):
    rec = 0
    while C[i+rec] == 'x' and rec < D and i+rec < 14:
        rec += 1
    if rec > 0:
        ans = max(ans, goldenWeek(C[:i] + 'o' * rec + C[i+rec]))
print(ans)