d = int(input())
a = list(input())
b = list(input())
week = a+b

schedule = []
now = 0
for w in week:
    if w == "o":
        if now > 0:
            now += 1
        else:
            schedule.append(now)
            now = 1
    else:
        if now < 0:
            now -= 1
        else:
            schedule.append(now)
            now = -1
schedule.append(now)
schedule = schedule[1:]

ans = 0
for i,v in enumerate(schedule):
    if v > 0:
        continue
    else:
        if abs(v) <= d:
            tmp = abs(v)
            try:
                tmp += schedule[i-1]
            except IndexError:
                continue
            try:
                tmp += schedule[i+1]
            except IndexError:
                continue
            ans = max(ans, tmp)
        else:
            tmp = d
            try:
                tmp += schedule[i-1]
            except IndexError:
                continue
            ans = max(ans, tmp)
            
            tmp = d
            try:
                tmp += schedule[i+1]
            except IndexError:
                continue
            ans = max(ans, tmp)
if not ans:
    ans = d
if schedule[0] > 0:
    ans = max(ans, d+schedule[0])
if schedule[-1] > 0:
    ans = max(ans, d+schedule[-1])
print(ans)