N = int(input())
S = input()
T = input()
S_ = []; T_ = []
if not (S[0] == T[0] and S[-1] == T[-1]):
    print(-1)
    exit()
for i in range(N):
    s = 1 if S[i] == 'A' else 0
    t = 1 if T[i] == 'A' else 0
    S_.append(s); T_.append(t)

S_diff = []; T_diff = []
for i in range(N - 1):
    s = S_[i] ^ S_[i + 1]
    t = T_[i] ^ T_[i + 1]
    S_diff.append(s); T_diff.append(t)

S_inv = []; T_inv = []
for i in range(N - 1):
    if i & 1:
        S_diff[i] = 1 - S_diff[i]
        T_diff[i] = 1 - T_diff[i]
    if S_diff[i] == 1:
        S_inv.append(i)
    if T_diff[i] == 1:
        T_inv.append(i)

if len(S_inv) != len(T_inv):
    print(-1)
    exit()
print(sum([abs(s - t) for s, t in zip(S_inv, T_inv)]))
exit()