'''
Created on 2015/06/06
'''

D = int(input())
date = "xxxxxxx" + input() + input() + "xxxxxxx"
def calc(day):
    ans, tmp = 0, 0
    for d in day:
        if d == 'o':
            tmp += 1
        else:
            ans = max(ans, tmp)
            tmp = 0
    return max(ans, tmp)

ans = calc(date)
for i in range(len(date)):
    d = date
    for x in range(i, len(date)):
        if x-i == D:
            break
        if date[x] == 'x':
            d = d[:x] + "o" + d[x+1:]
        else:
            break
    ans = max(ans, calc(d))

print(ans)