D = int(input())
part1 = input().strip()
part2 = input().strip()
calendar = part1 + part2
days = [c == 'o' for c in calendar]

# Compute left (consecutive o's ending at i) and right (consecutive o's starting at i)
n = len(days)
left = [0] * n
current = 0
for i in range(n):
    if days[i]:
        current += 1
    else:
        current = 0
    left[i] = current

right = [0] * n
current = 0
for i in range(n-1, -1, -1):
    if days[i]:
        current += 1
    else:
        current = 0
    right[i] = current

max_original = max(left + right)

# Candidate for front application (D days before the input)
candidate_front = 0
if days[0]:
    candidate_front = D + right[0]

# Candidate for back application (D days after the input)
candidate_back = 0
if days[-1]:
    candidate_back = left[-1] + D

# Find all x-ranges in the input days
x_ranges = []
i = 0
while i < n:
    if not days[i]:
        start = i
        while i < n and not days[i]:
            i += 1
        end = i - 1
        x_ranges.append((start, end))
    else:
        i += 1

max_x_candidate = 0
for (start, end) in x_ranges:
    L = end - start + 1
    A = 0
    if start > 0 and days[start - 1]:
        A = left[start - 1]
    B = 0
    if end < n - 1 and days[end + 1]:
        B = right[end + 1]
    used = min(D, L)
    current_candidate = A + used + B
    if current_candidate > max_x_candidate:
        max_x_candidate = current_candidate

max_total = max(max_original, candidate_front, candidate_back, max_x_candidate)
print(max_total)