import sys

n = int(sys.stdin.readline())
s = list(sys.stdin.readline().strip())
t = list(sys.stdin.readline().strip())

if s[0] != t[0] or s[-1] != t[-1]:
    print(-1)
    sys.exit()

count = 0

for j in range(1, n - 1):
    if s[j] == t[j]:
        continue
    # Need to flip s[j]
    left = s[j-1]
    right = s[j+1]
    if left == right:
        s[j] = 'B' if s[j] == 'A' else 'A'
        count += 1
    else:
        if j + 2 >= n:
            print(-1)
            sys.exit()
        if s[j] != s[j + 2]:
            print(-1)
            sys.exit()
        # Flip j+1
        s[j + 1] = 'B' if s[j + 1] == 'A' else 'A'
        count += 1
        # Check new right
        new_right = s[j + 1]
        if left == new_right:
            s[j] = 'B' if s[j] == 'A' else 'A'
            count += 1
        else:
            print(-1)
            sys.exit()

# Final check to ensure all characters match
if ''.join(s) == ''.join(t):
    print(count)
else:
    print(-1)