# 解説を読んだ

N=int(input())
S=input().strip()
T=input().strip()

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

if S==T:
    print(0)
    exit()

S=list(S)
T=list(T)

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

S2=[0]*(N-1)
T2=[0]*(N-1)

for i in range(N-1):
    S2[i]=S[i]^S[i+1]
    T2[i]=T[i]^T[i+1]

for i in range(N-1):
    if i%2==0:
        S2[i]^=1
        T2[i]^=1

SL=[]
TL=[]

for i in range(N-1):
    if S2[i]==0:
        SL.append(i)
    if T2[i]==0:
        TL.append(i)

if len(SL)!=len(TL):
    print(-1)
else:
    ANS=0

    for i in range(len(SL)):
        ANS+=abs(SL[i]-TL[i])

    print(ANS)