D=int(raw_input())
C1=raw_input()
C2=raw_input()
C="xxxxxxxxxxxxxx"+C1+C2+"xxxxxxxxxxxxxx"

ans=0
for i in range(len(C)):
    CurC=list(C)
    cur=0
    for j in range(D):
        if i+j >= len(C):
            break
        if CurC[i+j]=='o':
            break
        else:
            CurC[i+j]='o'
    for c in CurC:
        if c=='o':
            cur+=1
            ans=max(ans,cur)
        else:
            cur=0
print ans