N = int(input())
S = [ord(a) - 65 for a in input()]
T = [ord(a) - 65 for a in input()]
if S[0] != T[0] or S[-1] != T[-1]:
    print(-1)
    exit()

before, flip, ans = 0, 0, 0
for i in range(N - 1):
    xor = S[i] ^ T[i]
    next_xor = S[i+1] ^ T[i+1]
    r_before = before ^ xor ^ (flip & 1) ^ (S[i-1] ^ S[i+1]) if flip else 0
    next_flip = max(0, flip + r_before - 1)
    if (next_flip ^ next_xor) % 2:
        next_flip += 1

    before = r_before ^ 1 if flip and next_flip else 0
    flip = next_flip
    ans += flip

print(-1 if flip else ans)