n = int(input())
S = input()
T = input()
if S[0] != T[0] or S[-1] != T[-1]:
    print(-1)
    exit()
S = [ord(s)-ord("A") for s in S]
T = [ord(t)-ord("A") for t in T]

S2 = []
T2 = []

for i in range(n-1):
    S2.append(S[i]^S[i+1])
    T2.append(T[i]^T[i+1])

if sum(S2) != sum(T2):
    print(-1)
    exit()

for i in range(1,n-1,2):
    S2[i] ^= 1
    T2[i] ^= 1

Sone = []
Tone = []
for i in range(n-1):
    if S2[i]:
        Sone.append(i)
    if T2[i]:
        Tone.append(i)

ans = 0
for s,t in zip(Sone,Tone):
    ans += abs(s-t)
print(ans)