n = int(raw_input())
w = raw_input()+raw_input()

sall=[]
ball=[]
s = 0
b = 0

for e in w:
    if e=='o':
        if b>0:
            ball.append(b)
        b=0
        s+=1
    else:
        if s>0:
            sall.append(s)
        s=0
        b+=1

sall.append(s)
ball.append(b)

all=[]

for i in xrange(len(sall)-1):
    all.append(sall[i]+sall[i+1])

anss=[]

for i,e in enumerate(ball):
    if e<=n and i<len(all):
        anss.append(e+all[i])

print max(anss)