codevs2822

#include<iostream>
#include<algorithm>
#include<vector>
#include<queue>
#include<cstdio>
#include<cstring>
#define maxn 10005
using namespace std;
struct node
{
    int x;
    int y;
}a[maxn];
struct Edge
{
    int to;
    int next;
}edge[maxn];
int low[maxn];
int dfn[maxn];
int instack[maxn];
int visit[maxn];
int sccno[maxn];
int outdegree[maxn];
int cnt;
int step;
int indexx;
int scc_cnt;
int head[maxn];
vector<int>scc[maxn];
void add(int u,int v)
{
    edge[cnt].next=head[u];
    edge[cnt].to=v;
    head[u]=cnt++;
    return;
}
void tarjan(int u)
{
    low[u]=dfn[u]=++step;
    instack[++indexx]=u;
    visit[u]=1;
    for(int i=head[u];i!=-1;i=edge[i].next)
    {
        if(!dfn[edge[i].to])
        {
            tarjan(edge[i].to);
            low[u]=min(low[u],low[edge[i].to]);
        }
        else if(visit[edge[i].to])
        {
            low[u]=min(low[u],dfn[edge[i].to]);
        }
    }
    if(low[u]==dfn[u])
    {
        scc_cnt++;
        scc[scc_cnt].clear();
        do
        {
            scc[scc_cnt].push_back(instack[indexx]);
            sccno[instack[indexx]]=scc_cnt;
            visit[instack[indexx]]=0;
            indexx--;
        }
        while(u!=instack[indexx+1]);
    }
    return;
}
void init()
{
    memset(head,-1,sizeof(head));
    cnt=step=indexx=scc_cnt=0;
    memset(visit,0,sizeof(visit));
    memset(low,0,sizeof(low));
    memset(dfn,0,sizeof(dfn));
    return ;
}
int main()
{
    int n,m;
    int x,y;
    init();
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++)
    {
        scanf("%d%d",&a[i].x,&a[i].y);
        add(a[i].x,a[i].y);
    }
    for(int i=1;i<=n;i++)
        if(!dfn[i])
        tarjan(i);
    for(int i=1;i<=n;i++)
    {
        if(sccno[a[i].x]!=sccno[a[i].y])
            outdegree[sccno[a[i].x]]++;
    }
    int flag=0;
    int pos;
    for(int i=1;i<=scc_cnt;i++)
    {
        if(outdegree[i]==0)
        {
            flag++;pos=i;
        }
    }
    int ans=0;
    for(int i=1;i<=scc_cnt;i++)
    {
        if(scc[i].size()>1)
            ans++;
    }
    cout<<ans<<endl;
        if(flag==1)
        {
            if(scc[pos].size()>1)
            {
                sort(scc[pos].begin(),scc[pos].end());
                for(int i=0;i<scc[pos].size();i++)
                    cout<<scc[pos][i]<<" ";
            }
            else
                cout<<"-1\n";
        }
        else
            cout<<"-1\n";
    return 0;
}