n = int(input())
s = input()
t = input()

if s[0] != t[0] and s[-1] != t[-1]:
    print(-1)
    exit()

a = "A"
s = [(a==s[i])^(a==s[i+1]) for i in range(n-1)]
t = [(a==t[i])^(a==t[i+1]) for i in range(n-1)]

for i in range(0, n-1, 2):
    s[i] ^= 1
    t[i] ^= 1

a = [i for i in range(n-1) if s[i] == 1]
b = [i for i in range(n-1) if t[i] == 1]

if len(a) != len(b):
    print(-1)
    exit()

print(sum(abs(a[i]-b[i]) for i in range(len(a))))