from itertools import *
from bisect import *
# from math import *
from collections import *
from heapq import *
from random import *
from decimal import *
import numpy as np
import sys

sys.setrecursionlimit(10 ** 6)
int1 = lambda x: int(x) - 1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def MI1(): return map(int1, sys.stdin.readline().split())
def MF(): return map(float, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LF(): return list(map(float, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
dij = [(0, 1), (1, 0), (0, -1), (-1, 0)]

def main():
    d = II()
    s = input() + input() + "x"
    co = cx = 0
    ans = 0
    pco = 0
    for c in s:
        if c == "o":
            co += 1
        else:
            if co > 0:
                if cx > d or pco == 0:
                    ans = max(ans, co + d, pco + d)
                else:
                    ans = max(ans, pco + cx + co)
                pco = co
                co = 0
                cx = 1
            else:
                cx += 1
    ans = max(ans, pco + d)
    print(ans)

main()