#!/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
import typing
class LazySegTree:
    def __init__(self, op, e, mapping, composition, _id, v): #v:初期配列
        self._op = op
        self._e = e
        self._mapping = mapping
        self._composition = composition
        self._id = _id
 
        if isinstance(v, int):
            v = [e]*v
 
        self._n = len(v)
        self._log = (self._n-1).bit_length()
        self._size = 1 << self._log
        self._d = [self._e]*(2*self._size)
        self._lz = [self._id]*self._size
        for i in range(self._n):
            self._d[self._size+i] = v[i]
        for i in range(self._size-1, 0, -1):
            self._update(i)
 
    def set(self, p, x):
        p += self._size
        for i in range(self._log, 0, -1):
            self._push(p >> i)
        self._d[p] = x
        for i in range(1, self._log+1):
            self._update(p >> i)
 
    def get(self, p):
        p += self._size
        for i in range(self._log, 0, -1):
            self._push(p >> i)
        return self._d[p]
 
    def prod(self, left, right):
        if left == right:
            return self._e
 
        left += self._size
        right += self._size
 
        for i in range(self._log, 0, -1):
            if ((left >> i) << i) != left:
                self._push(left >> i)
            if ((right >> i) << i) != right:
                self._push(right >> i)
 
        sml = self._e
        smr = self._e
        while left < right:
            if left & 1:
                sml = self._op(sml, self._d[left])
                left += 1
            if right & 1:
                right -= 1
                smr = self._op(self._d[right], smr)
            left >>= 1
            right >>= 1
 
        return self._op(sml, smr)
 
    def all_prod(self):
        return self._d[1]
 
    def apply(self, left, right, f):
        if right is None:
            p = left
            p += self._size
            for i in range(self._log, 0, -1):
                self._push(p >> i)
            self._d[p] = self._mapping(f, self._d[p])
            for i in range(1, self._log+1):
                self._update(p >> i)
        else:
            if left == right:
                return
 
            left += self._size
            right += self._size
 
            for i in range(self._log, 0, -1):
                if ((left >> i) << i) != left:
                    self._push(left >> i)
                if ((right >> i) << i) != right:
                    self._push((right-1) >> i)
 
            l2 = left
            r2 = right
            while left < right:
                if left & 1:
                    self._all_apply(left, f)
                    left += 1
                if right & 1:
                    right -= 1
                    self._all_apply(right, f)
                left >>= 1
                right >>= 1
            left = l2
            right = r2
 
            for i in range(1, self._log+1):
                if ((left >> i) << i) != left:
                    self._update(left >> i)
                if ((right >> i) << i) != right:
                    self._update((right-1) >> i)
 
    def max_right(self, left, g):
        if left == self._n:
            return self._n
 
        left += self._size
        for i in range(self._log, 0, -1):
            self._push(left >> i)
 
        sm = self._e
        first = True
        while first or (left & -left) != left:
            first = False
            while left%2 == 0:
                left >>= 1
            if not g(self._op(sm, self._d[left])):
                while left < self._size:
                    self._push(left)
                    left *= 2
                    if g(self._op(sm, self._d[left])):
                        sm = self._op(sm, self._d[left])
                        left += 1
                return left-self._size
            sm = self._op(sm, self._d[left])
            left += 1
 
        return self._n
 
    def min_left(self, right, g):
        if right == 0:
            return 0
 
        right += self._size
        for i in range(self._log, 0, -1):
            self._push((right-1) >> i)
 
        sm = self._e
        first = True
        while first or (right & -right) != right:
            first = False
            right -= 1
            while right > 1 and right%2:
                right >>= 1
            if not g(self._op(self._d[right], sm)):
                while right < self._size:
                    self._push(right)
                    right = 2*right+1
                    if g(self._op(self._d[right], sm)):
                        sm = self._op(self._d[right], sm)
                        right -= 1
                return right+1-self._size
            sm = self._op(self._d[right], sm)
 
        return 0
 
    def _update(self, k):
        self._d[k] = self._op(self._d[2*k], self._d[2*k+1])
 
    def _all_apply(self, k, f):
        self._d[k] = self._mapping(f, self._d[k])
        if k < self._size:
            self._lz[k] = self._composition(f, self._lz[k])
 
    def _push(self, k):
        self._all_apply(2*k, self._lz[k])
        self._all_apply(2*k+1, self._lz[k])
        self._lz[k] = self._id
 
# treeのマージ
# 32bit以上を区間長とした例
def op(x, y):
    return x+y

# lazy(f)からtree(x)への操作
def mapping(f, x):
    if f == -1:return x
    return f ^ x
 
# lazyの下への分解
def composition(f, g):
    if f == -1:return g
    if g == -1:return f
    return f ^ g
 
# lazyの単位元
_id = -1
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

lst = LazySegTree(op,0,mapping,composition,_id,n)
for i in range(1,n-1):
    fl = lst.get(i)
    if (fl == 0 and s[i] != t[i]) or (fl == 1 and s[i] == t[i]):
        #print(i)
        j = max(j,i + 1)
        while j < n and ((lst.get(j-2) == 0 and s[j-2] != s[j]) or (lst.get(j-2) == 1 and s[j-2] == s[j])):
            j += 1
        if j == n:
            print(-1)
            break
        lst.apply(i,j,1)
        ans += j - i
        j += 1
else:
    print(ans)