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()


flag_s=0
flag_t=0
for i in range(N-2):
    if S[i]==S[i+2]:
        flag_s=1

    if T[i]==T[i+2]:
        flag_t=1

if flag_s==1 and flag_t==1:
    ANS=0
    for i in range(N):
        if S[i]!=T[i]:
            ANS+=1

    print(ANS)
else:
    print(-1)