d = int(input())
week = list(input())+list(input())
maxholi = 0

def search(i,hol,cnt):
    global maxholi
    if i==14:
        maxholi = max(maxholi,cnt)
        return
    
    if week[i]=="o":
        cnt += 1
    else:
        if hol>0:
            search(i+1,hol-1,cnt+1)
        maxholi = max(maxholi,cnt)
        cnt = 0
    search(i+1,hol,cnt)

search(0,d,0)
print(maxholi)