from collections import deque

def Manacher(S):
    N=len(S)
    manacher=[None]*N
    i,j=0,0
    while i<N:
        while 0<=i-j and i+j<N and S[i-j]==S[i+j]:
            j+=1
        manacher[i]=j
        k=1
        while 0<=i-k and k+manacher[i-k]<j:
            manacher[i+k]=manacher[i-k]
            k+=1
        i+=k
        j-=k
    return manacher

N,M=map(int,input().split())
S=input()
T=input()
if T==T[::-1] and M%2==0:
    TT=["."]
    for t in T:
        TT.append(t)
        TT.append(".")
    manacher=[(c-1)//2 for c in Manacher(TT)[::2]]
    palindrome=[i for i in range(M+1)]
    r=0
    for i in range(M+1):
        while r<=i+manacher[i]:
            palindrome[r]=i
            r+=1
    inf=1<<30
    ans=inf
    if S==T:
        ans=0
    for _ in range(2):
        le=0
        for s,t in zip(S,T):
            if s==t:
                le+=1
            else:
                break
        cnt=0
        cur=M
        while True:
            if le*2>=cur:
                ans=min(ans,cnt+1)
                break
            if cur<=palindrome[cur//2]*2:
                break
            cur=palindrome[cur//2]*2
            cnt+=1
        S=S[::-1]
    if ans==inf:
        ans=-1
else:
    ans=-1
print(ans)