import re
d = int(input())
W1 = input()
W2 = input()
GW = W1 + W2

pat = re.compile('o+')
ans = [0]
for i in range(14-d):
    s = GW[:i] + 'o'*d + GW[i+d:]
    match = pat.findall(s)
    if match:
        ans.append(max([len(x) for x in match]))
    
print(max(ans))