D = input()
cal = 'x'*D + raw_input() + raw_input() + 'x'*D

def max_successive_holidays(cal):
    cal = cal.split('x')
    holidays = [len(item) for item in cal]
    return max(holidays)

max_holidays = 0
for i in xrange(len(cal)-D):
    if 'ox' not in cal[i:i+D]:
        cal_days_off = list(cal)
        cal_days_off[i:i+D] = ['o'] * D
        cal_days_off = ''.join(cal_days_off)
    max_holidays = max(max_successive_holidays(cal_days_off), max_holidays)

print max_holidays