D=int(raw_input())
C1=raw_input()
C2=raw_input()
C=C1+C2
ans=0
for i in range(len(C)):
    cur=0
    for j,c in enumerate(C):
        if c=='o' or (i<=j and j<i+D):
            cur+=1
            ans=max(ans,cur)
        else:
            cur=0
print ans