import sys
readline=sys.stdin.readline

N=int(readline())
S=[int(s=="A") for s in readline().rstrip()]
T=[int(t=="A") for t in readline().rstrip()]
if S[0]!=T[0]:
    print(-1)
    exit()
S=[S[i]^S[i+1] for i in range(N-1)]
T=[T[i]^T[i+1] for i in range(N-1)]
for i in range(0,N-1,2):
    S[i]^=1
    T[i]^=1
if S.count(1)!=T.count(1):
    print(-1)
    exit()
ans=sum(abs(i-j) for i,j in zip([i for i in range(N-1) if S[i]],[j for j in range(N-1) if T[j]]))
print(ans)