import sys
from collections import deque
input=sys.stdin.readline
ii=lambda:int(input())
n=ii()
s=list(input().strip())
t=list(input().strip())
d=[0]*n
for i in range(n):
    d[i]=s[i]!=t[i]
if d[0] or d[-1]:
    print(-1)
    sys.exit()
q=deque()
for i in range(1,n-1):
    if d[i] and s[i-1]==s[i+1]:
        q.append(i)
ans=0
while q:
    i=q.popleft()
    if not d[i] or s[i-1]!=s[i+1]:
        continue
    d[i]=0
    ans+=1
    s[i]='A' if s[i]=='B' else 'B'
    for k in (i-1,i+1):
        if 1<=k<n-1 and d[k] and s[k-1]==s[k+1]:
            q.append(k)
for v in d:
    if v:
        print(-1)
        sys.exit()
print(ans)