"""

2018:
基本的に可逆
端は不変

最初と最後に、
A?A,B?B
が或るのは必須

反転しなければ行けない箇所に色を付けよう


"""

import sys
from sys import stdin

def change(l,r,L,R):

    ll = min(l,L)
    rr = max(r,R)

    d = r-l+1
    D = R-L+1
    dd = rr-ll+1
    
    return dd*2 - d - D


def getone(lis):

    ret = []

    for i in range(N):
        if lis[i] == 1:
            if len(ret) == 0 or ret[-1][1]+1 != i:
                ret.append( [i,i] )
            else:
                ret[-1][1] += 1

    return ret
                

N = int(stdin.readline())

S = list(stdin.readline()[:-1])
T = list(stdin.readline()[:-1])

for i in range(N):
    if S[i] == "A":
        S[i] = 0
    else:
        S[i] = 1

for i in range(N):
    if T[i] == "A":
        T[i] = 0
    else:
        T[i] = 1


if S[0] != T[0] or S[-1] != T[-1]:
    print (-1)
    sys.exit()


S01 = 0
T01 = 0

for i in range(N-1):
    if S[i] == 0 and S[i+1] == 1:
        S01 += 1
    if T[i] == 0 and T[i+1] == 1:
        T01 += 1

if S01 != T01:
    print (-1)
    sys.exit()

SG = getone(S)
TG = getone(T)

ans = 0
for i in range(len(SG)):

    ans += change(SG[i][0],SG[i][1],TG[i][0],TG[i][1])

print (ans)