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

diff_positions = []
for i in range(n):
    if s[i] != t[i]:
        diff_positions.append(i)

count = 0
possible = True

for i in diff_positions:
    if i < 1 or i >= n-1:
        possible = False
        break
    if s[i-1] != s[i+1]:
        possible = False
        break
    # Perform the operation
    s[i] = 'B' if s[i] == 'A' else 'A'
    count += 1

if possible and s == t:
    print(count)
else:
    print(-1)