#include<bits/stdc++.h>
using namespace std;
const int N=1e6+10;
#define PII pair<int,int>
vector<PII>G[N];
stack<PII>stk;int v[N],instk[N];
void dfs(int x)
{
	if(instk[x])
	{
		int z=-1;deque<int>q;
		while(z!=x)
		{
			int y=stk.top().second;z=stk.top().first;stk.pop();
			q.push_front(y-1);
		}
		cout<<q.size()<<'\n';
		for(int i:q)cout<<i<<'\n';
		exit(0);
	}
	if(v[x])return;v[x]=1;
	for(auto i:G[x])
	{
		int y=i.first,w=i.second;
		stk.push({x,w});instk[x]=1;
		dfs(y);
		stk.pop();instk[x]=0;
	}
}
int main()
{
	int n,m;cin>>n>>m;
	for(int i=1;i<=m;i++)
	{
		int x,y;cin>>x>>y;x++,y++;
		G[x].push_back({y,i});
	}
	for(int i=1;i<=n;i++)if(!v[i])
	{
		while(!stk.empty())stk.pop();
		dfs(i);
	}
	puts("-1");
	return 0;
}