#!/usr/bin/env PyPy3

from collections import Counter, defaultdict, deque
import itertools
import re
import math
from functools import reduce
import operator
import bisect
from heapq import *
import functools
mod=998244353

import sys
input=sys.stdin.readline

n = int(input())
s = list(input().rstrip())
t = list(input().rstrip())
if s[0] != t[0] or s[-1] != t[-1]:
    print(-1)
    exit()
ans = 0
fl = 0
j = 0
li = [0] * n
fll = [0] * n
for i in range(1,n-1):
    if li[i]:
        fl = 1 - fl
    fll[i] = fl
    if (fl == 0 and s[i] != t[i]) or (fl == 1 and s[i] == t[i]):
        j = max(j,i + 1)
        while j < n and ((fll[j-2] == 0 and s[j-2] != s[j]) or (fll[j-2] == 1 and s[j-2] == s[j])):
            j += 1
        if j == n:
            print(-1)
            break
        li[j] += 1
        ans += j - i
        j += 1
        fl = 1 - fl
    #print(fl,i)
    fll[i] = fl
else:
    print(ans)