n=int(input())
s=[*input()]
t=[*input()]
ans=0
for i in range(n-2)[::2]:
    if s[i]==s[i+2]:
        if s[i+1]!=t[i+1]:
            s[i+1]=t[i+1]
            ans+=1
for i in range(1,n-2)[::2]:
    if s[i]==s[i+2]:
        if s[i+1]!=t[i+1]:
            s[i+1]=t[i+1]
            ans+=1
if s==t:print(ans)
else:print(-1)