# pythonしょしんしゃ

d = int(input())
ll = "xxxxxxxxxxxxxxx" + input() + input() + "xxxxxxxxxxxxxxx"

result = d
for x in range(1,30):
    fail = False
    for i in range(x, x + d):
        if ll[x] == 'o':
            fail = True
            break
        # l[x] = 'o' # むり
    if fail: continue
    
    l = ll[:x]+('o'*d)+ll[x+d:]
    # print(l)
    
    nice = 0
    for c in l:
        if c == 'o':
            nice+=1
            result = max(result, nice)
        else:
            nice = 0

print(result)