n = int(input())
S = input()
T = input()
if S[0] != T[0] or S[-1] != T[-1]:
    print(-1)
    exit()

def f(S):
    lst = []
    for i in range(n - 1):
        if S[i] == S[i + 1]:
            lst.append(1)
        else:
            lst.append(0)
    for i in range(1, n - 1, 2):
        lst[i] ^= 1
    return lst

S = f(S)
T = f(T)
if S.count(1) != T.count(1):
    print(-1)
    exit()

def g(S):
    lst = []
    for i, s in enumerate(S):
        if s == 1:
            lst.append(i)
    return lst

S = g(S)
T = g(T)
print(sum(abs(s - t) for s, t in zip(S, T)))