class SegmentTree:
    def __init__(self, init_val, segfunc, ide_ele):
        n = len(init_val)
        self.segfunc = segfunc
        self.ide_ele = ide_ele
        self.num = 1 << (n - 1).bit_length()
        self.tree = [ide_ele] * 2 * self.num
        self.size = n
        for i in range(n):
            self.tree[self.num + i] = init_val[i]
        for i in range(self.num - 1, 0, -1):
            self.tree[i] = self.segfunc(self.tree[2 * i], self.tree[2 * i + 1])

    def update(self, k, x):
        k += self.num
        self.tree[k] = x
        while k > 1:
            k >>= 1
            self.tree[k] = self.segfunc(self.tree[2*k], self.tree[2*k+1])

    def query(self, l, r):
        if r==self.size:
            r = self.num

        res = self.ide_ele

        l += self.num
        r += self.num
        right = []
        while l < r:
            if l & 1:
                res = self.segfunc(res, self.tree[l])
                l += 1
            if r & 1:
                right.append(self.tree[r-1])
            l >>= 1
            r >>= 1

        for e in right[::-1]:
            res = self.segfunc(res,e)
        return res

from math import radians
import sys,random

input = lambda :sys.stdin.readline().rstrip()
mi = lambda :map(int,input().split())
li = lambda :list(mi())

"""
from:https://tjkendev.github.io/procon-library/python/string/manacher.html
"""
# 偶数長含めた回文の長さを求める
# R[2*i] = L: S[i]を中心とする奇数長の最大回文
# R[2*i+1] = L: S[i:i+2]を中心とする偶数長の最大回文
# ダミー文字を挟むが、各 R[i] は実際の回文の文字列長と一致する
def manacher(S):
    C = []
    for a in S:
        C.append(a)
        C.append(0)
    C.pop()

    L = len(C)

    R = [0]*L

    i = j = 0
    while i < L:
        while j <= i < L-j and C[i-j] == C[i+j]:
            j += 1
        R[i] = j
        k = 1
        while j-R[i-k] > k <= i < L-k:
            R[i+k] = R[i-k]
            k += 1
        i += k; j -= k
    return R


N,M = mi()
S = input()
T = input()

if M&1:
    exit(print(-1))
if any(T[i]!=T[-i-1] for i in range(M)):
    exit(print(-1))

R = manacher(T)

m = M//2
INF = 10**9
dp = [INF for i in range(M)]
dp[-1] = 0
dp = SegmentTree([INF]*M,min,INF)
dp.update(M-1,0)
for i in range(M-2,-1,-1):
    l = i
    r = i + (R[2*i+1]//2)
    tmp = dp.query(l+1,r+1) + 1
    if tmp < INF:
        dp.update(i,tmp)

dp = dp.tree[dp.num:]

res = INF
for i in range(N):
    if S[i]==T[i]:
        res = min(res,dp[i])
    else:
        break
for i in range(N-1,-1,-1):
    if S[i]==T[M-1-(N-1-i)]:
        res = min(res,dp[N-1-i])
    else:
        break

if res==INF:
    print(-1)
else:
    print(res)