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