n = int(input())
s = list(input())
t = list(input())
count = 0

for i in range(n - 2):
    # Check positions from 1 to N-2 (0-based 0 to N-3)
    if s[i] == s[i + 2]:
        if s[i + 1] != t[i + 1]:
            # Flip the character
            s[i + 1] = 'A' if s[i + 1] == 'B' else 'B'
            count += 1

# Check the remaining positions (N-2 to N-1) might need to check again
# After processing, check if s equals t
if s == t:
    print(count)
else:
    print(-1)