#golden week
D=int(input())
C1=input()
C2=input()
C=C1+C2
i=0
Cn=[]
while i<len(C):
    co=0
    while i <len(C) and C[i]=="o":
        co+=1
        i+=1
    Cn.append (co)
    co=0
    while i <len(C) and C[i]=="x":
        co+=1
        i+=1
    Cn.append (co)
m=0
j=0
#print (Cn)
while j<len(Cn):
    V=D
    i=j
  #  print (i)
    g=0
    while i<len(Cn) and V>=0:
        if i%2==0:
            g+=Cn[i]
     #       print (g)
        elif V>=Cn[i]:
            g+=Cn[i]
            V-=Cn[i]
        elif V>0:
            g+=V
            V=0
        else:
            V=-1
        i+=1
    #    print (D)
 #   print (g)
    if g>m:
        m=g
    j+=1
print(m)