D = int(input())
c1 = input().strip()
c2 = input().strip()
arr = list(c1 + c2)

max_original = 0
current = 0
for c in arr:
    if c == 'o':
        current += 1
        if current > max_original:
            max_original = current
    else:
        current = 0

if D == 0:
    print(max_original)
    exit()

max_total = 0

for i in range(0, 14 - D + 1):
    valid = True
    for j in range(i, i + D):
        if j >= 14 or arr[j] != 'x':
            valid = False
            break
    if valid:
        left = 0
        if i > 0:
            pos = i - 1
            while pos >= 0 and arr[pos] == 'o':
                left += 1
                pos -= 1
        
        right = 0
        if i + D < 14:
            pos = i + D
            while pos < 14 and arr[pos] == 'o':
                right += 1
                pos += 1
        
        total = left + D + right
        if total > max_total:
            max_total = total

result = max(max_original, max_total)
print(result)