from collections import deque
def main():
    n = int(input())
    S = input()
    T = input()
    if S[0] != T[0] or S[-1] != T[-1]: return -1
    A = [1 if S[i]!=S[i+1] else 0 for i in range(n-1)]
    B = [1 if T[i]!=T[i+1] else 0 for i in range(n-1)]
    dq = deque([])
    ans = 0
    for i in range(n-1):
        if i and A[i] == A[i-1]:
            if dq:
                j = dq.popleft()
                ans += i-j
                A[j] ^= 1
                A[i] ^= 1
        if A[i] != B[i]:
            dq.append(i)
    if A == B:
        return ans
    else:
        return -1

print(main())