n = int(input())
S = input()
T = input()

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

def ok(S):
    for i in range(1, n - 1):
        if S[i - 1] == S[i + 1]:
            return True
    return False

if not ok(S) or not ok(T):
    print(-1)
else:
    print(sum(s != t for s, t in zip(S, T)))