d = int(input())
s = ''
for _ in range(2):
    s += input()

C = []
x = d
o = 0
for i in range(14):
    if s[i] == 'x':
        x += 1
        if o > 0 and i != 0:
            C.append(o)
        o = 0
    else:
        o += 1
        if x > 0 or i == 0:
            C.append(x)
        x = 0
    if i == 13:
        if x > 0:
            C.append(x + d)
            C.append(0)
        else:
            C.append(o)
            C.append(d)
            C.append(0)
n = len(C)
ans = 0
i = 0
for i in range(0, n, 2):
    if i == 0:
        x, o = C[i:i + 2]
        ans = max(ans, min(x, d) + o)
    else:
        o1, x, o2 = C[i - 1:i + 2]
        if x <= d:
            ans = max(ans, o1 + x + o2)
        else:
            ans = max(ans, o1 + d, o2 + d)
print(ans)