#include<cstdio>

#include<cstring>

#include<cmath>

#include<queue>

#include<algorithm>

#define maxn 40005

#define maxm 300005

#define INF 0x3f3f3f3f

using namespace std;

inline int getint()

{

    int num=,flag=;char c;

    while((c=getchar())<''||c>'')if(c=='-')flag=-;

    while(c>=''&&c<='')num=num*+c-,c=getchar();

    return num*flag;

}

int n,K,S,T;

int fir[maxn],nxt[maxm],to[maxm],cap[maxm],cnt;

int h[maxn],tp[maxn];

int pri[maxn],np[maxn],cur;

int A,B;

int num[maxn],ans[maxn],vis[maxn],pos[maxn];

inline void newnode(int u,int v,int w)

{to[++cnt]=v,nxt[cnt]=fir[u],fir[u]=cnt,cap[cnt]=w;}

inline void insert(int u,int v,int w)

{newnode(u,v,w),newnode(v,u,);}

inline bool bfs()

{

    memset(h,-,sizeof h);

    queue<int>Q;h[S]=,Q.push(S);

    while(!Q.empty())

    {

        int u=Q.front();Q.pop();

        for(int i=fir[u];i;i=nxt[i])

            if(cap[i]&&!~h[to[i]])

            {

                h[to[i]]=h[u]+,Q.push(to[i]);

                if(to[i]==T)return ;

            }

    }

    return ;

}

inline int aug(int u,int flow)

{

    if(u==T)return flow;

    int used=;

    for(int i=tp[u];i;i=nxt[i])

    {

        tp[u]=i;

        if(cap[i]&&h[to[i]]==h[u]+)

        {

            int w=flow-used;

            w=aug(to[i],min(cap[i],w));

            cap[i]-=w,cap[i^]+=w,used+=w;

            if(used==flow)return flow;

        }

    }

    if(!used)h[u]=-;

    return used;

}

inline int dinic()

{

    int num=;

    while(bfs())memcpy(tp,fir,sizeof fir),num+=aug(S,INF);

    return num;

}

inline void init()

{

    np[]=;

    for(int i=;i<=n;i++)

    {

        if(!np[i])pri[++cur]=i;

        for(int j=;j<=cur&&i*pri[j]<=n;j++)

        {

            np[i*pri[j]]=;

            if(i%pri[j]==)break;

        }

    }

}

inline void dfs(int u)

{

    vis[u]=;

    for(int i=fir[u];i;i=nxt[i])if(cap[i]&&!vis[to[i]])dfs(to[i]);

}

int main()

{

    n=getint(),K=getint();

    init();

    for(A=;A<=cur;A++)if(n%pri[A]==)break;

    for(B=A+;B<=cur;B++)if(n%pri[B]==)break;

    if(n==){printf("-1\n");return ;}

    if(B>cur)

    {

        A=n/pri[A];

        for(int i=;i<=K;i++)

        {

            int x=getint();ans[x]=;

            if(!vis[x])for(int j=(x-)%A+;j<=n;j+=A)vis[j]=;

        }

        int num=;

        for(int i=;i<=n;i++)num+=vis[i];

        if(num==n){printf("-1\n");return ;}

        printf("%d\n",num-K);

        for(int i=,flag=;i<=n;i++)

            if(vis[i]&&!ans[i])

            {

                printf("%d",i);

                if((++flag)==num-K)printf("\n");

                else printf(" ");

            }

    }

    else

    {

        A=n/pri[A],B=n/pri[B];

        S=A+B+,T=S+,cnt=;

        for(int i=;i<=A;i++)num[i]=n/A;

        for(int i=;i<=B;i++)num[A+i]=n/B;

        for(int i=;i<=K;i++)

        {

            int x=getint();ans[x]=;

            int tmp1=(x-)%A+,tmp2=(x-)%B+A+;

            pos[tmp1]=pos[tmp2]=;

            num[tmp1]--,num[tmp2]--;

        }

        for(int i=;i<=n;i++)

        {

            int tmp1=(i-)%A+,tmp2=(i-)%B+A+;

            if(pos[tmp1]&&pos[tmp2])insert(tmp1,tmp2,INF);

        }

        for(int i=;i<=A;i++)if(pos[i])insert(S,i,num[i]);

        for(int i=A+;i<=A+B;i++)if(pos[i])insert(i,T,num[i]);

        int sum=dinic();

        if(sum==n-K){printf("-1\n");return ;}

        printf("%d\n",sum);

        dfs(S);

        for(int i=;i<=A;i++)if(pos[i]&&!vis[i])for(int j=i;j<=n;j+=A)ans[j]^=;

        for(int i=;i<=B;i++)if(pos[i+A]&&vis[i+A])for(int j=i;j<=n;j+=B)ans[j]^=;

        for(int i=,flag=;i<=n;i++)

            if(ans[i])

            {

                printf("%d",i);

                if((++flag)==sum){printf("\n");break;}

                else printf(" ");

            }

    }

}