n = int(input())
s = list(input().strip())
t = list(input().strip())

operations = 0

for i in range(n - 2):
    if s[i] != s[i + 2]:
        continue
    if s[i + 1] == t[i + 1]:
        continue
    # Perform the flip
    s[i + 1] = 'A' if s[i + 1] == 'B' else 'B'
    operations += 1

if s == t:
    print(operations)
else:
    print(-1)