import re
d = int(input())
W1 = input()
W2 = input()
GW = W1 + W2
pat = re.compile('o+')
pat2 = re.compile('x+')
ans = [0]

for i in range(14):
    mob =  pat2.search(GW, i)
    if mob:
        (j, k) = mob.span()
        if k - j < d:
            v = GW[:j] + 'o'*(k-j) + GW[k:]
            matchv = pat.findall(v)
            if matchv:
                ans.append(max([len(q) for q in matchv]))

for i in range(14-d):
    if 'o' not in GW[i:i+d]:
        s = GW[:i] + 'o'*d + GW[i+d:]
        match = pat.findall(s)
        if match:
            ans.append(max([len(x) for x in match]))
   
p_gw = 'o'*d + GW
p = pat.findall(p_gw)
if p:
    ans.append(max([len(y) for y in p]))
a_gw = GW + 'o'*d
a = pat.findall(a_gw)
if a:
    ans.append(max([len(z) for z in a]))

print(max(ans))