from collections import deque
def manacher(s):
    i, j = 0, 0
    n = len(s)
    ret = [0]*n
    while i < n:
        while i-j>=0 and i+j<n and s[i-j]==s[i+j]:
            j+=1
        ret[i] = j
        k = 1
        while i-k>=0 and k+ret[i-k]<j:
            ret[i+k]=ret[i-k]
            k+=1
        i+=k
        j-=k
    return ret

def manacher_even(s):
    n = len(s)
    ret = [0]*(n-1)
    ss = ['@']*(2*n)
    for i in range(n):
        ss[2*i] = s[i]
    m = manacher(ss)
    for i in range(n-1):
        ret[i] = m[2*i+1]//2
    return ret

N, M = map(int,input().split())
S = input()
T = input()

if M%2==1:
    print(-1)
    exit(0)
for i in range(M):
    if T[i]!=T[M-1-i]:
        print(-1)
        exit(0)
    

M//=2
T = T[0:M]
print(T)
INF = 1<<30
dp = [INF]*M
bfs = deque()
for i in range(N):
    if i>=M or S[i]!=T[i]:
        break
    dp[i] = 1
    bfs.append(i)

for i in range(N):
    if i>= M or S[N-1-i]!=T[i]:
        break
    if dp[i] == INF:
        bfs.append(i)
    dp[i] = 1

R = manacher_even(T)
G = [[]for i in range(M)]
for i in range(M-1):
    G[i+1].append((i,0))
    if R[i] > 0:
        G[i].append((i+R[i],1))

while len(bfs) > 0:
    v = bfs.popleft()
    for (nx, c) in G[v]:
        if dp[nx] > dp[v] + c:
            dp[nx] = dp[v] + c
            if c == 0:
                bfs.appendleft(nx)
            else:
                bfs.append(nx)

print(-1 if dp[M-1] == INF else dp[M-1])