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

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

D = [0] * n
for i in range(n):
    if s[i] != t[i]:
        D[i] = 1

count = 0
possible = True

for j in range(1, n-1):  # j is 0-based, from 1 to n-2 inclusive
    if D[j] == 1:
        if j == 1:
            # Check if S[0] == S[2]
            if s[0] != s[2]:
                possible = False
                break
        else:
            # Check if T[j-2] == original S[j]
            if t[j-2] != s[j]:
                possible = False
                break
        count += 1

if possible:
    print(count)
else:
    print(-1)