# 雑提出（通らない）
N = int(input())
S = [ord(a) - 65 for a in input()]
T = [ord(a) - 65 for a in input()]
inf = 10 ** 9
null, m, lm, rm, lrm, mlrm, rmlrm, lmlrm,lrmlrm = 0, inf, inf, inf, inf, inf, inf, inf, inf
for i in range(1, N - 1):
    nnull, nm, nlm, nrm, nlrm, nmlrm, nrmlrm, nlmlrm, nlrmlrm = inf, inf, inf, inf, inf, inf, inf, inf, inf
    if S[i] == T[i]:
        nnull = min(null, m, lm)
        if S[i-1] == S[i+1]:
            nmlrm = min(rm, lrm, rmlrm, lrmlrm) + 2
            nlrmlrm = min(mlrm, lmlrm) + 2
        else:
            nrmlrm = min(rm, lrm, rmlrm, lrmlrm) + 2
            nlmlrm = min(mlrm, lmlrm) + 2
    else:
        if S[i-1] == S[i+1]:
            nm = min(null, rm, lrm) + 1
            nlrm = min(m, lm, mlrm, lmlrm) + 1
        else:
            nlm = min(m, lm, mlrm, lmlrm) + 1
            nrm = min(null, rm, lrm) + 1
    null, m, lm, rm, lrm, mlrm, rmlrm, lmlrm, lrmlrm = nnull, nm, nlm, nrm, nlrm, nmlrm, nrmlrm, nlmlrm, nlrmlrm
ans = min((null, m, lm))
print(ans if ans < inf else -1)