#include<cstdio>
#include<vector>
using namespace std;

bool dfs(const vector<vector<int>> &g, vector<int> &vis, vector<bool> &hasCycle, int u)
{
	bool ret = false;
	vis[u] = -1;
	for(const int &v : g[u])
	{
		if(vis[v] == 0 && dfs(g, vis, hasCycle, v) )
		{
			ret = true;
		}
		else if(vis[v] == -1)
		{
			ret = true;
		}
		else if(vis[v] == 1 && hasCycle[v])
		{
			ret = true;
		}
	}
	vis[u] = 1;
	hasCycle[u] = ret;
	return ret;
}

int main()
{
	int n, m;
	vector<vector<int>> g;
	vector<int> vis;
	vector<bool> hasCycle;
	scanf("%d%d", &n, &m);
	g.resize(n);
	vis.resize(n, 0);
	hasCycle.resize(n, false);
	for(int i = 0; i < m; ++i)
	{
		int from, to;
		scanf("%d%d", &from, &to);
		g[from].emplace_back(to);
	}
	for(int i = 0; i < n; ++i)
	{
		if(vis[i] == 0)
		{
			dfs(g, vis, hasCycle, i);
		}
	}
	
	bool first = true;
	for(int i = 0; i < n; ++i)
	{
		if(hasCycle[i])
		{
			if(first == false) printf(" ");
			else first = false;
			printf("%d", i);
		}
	}
	if(first == true) printf("-1");
	printf("\n");
	return 0;
}