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#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
#include <stdlib.h>

using namespace std;

struct vertex
{
	int visitedId;
	int onCycleId;
	bool isInResult;
	bool isOnStack;
	set< int > links;

	vertex() : visitedId( -10000 ), onCycleId( -10000 ), isInResult( true ), isOnStack( false ) {}
};

vector< vertex > verts;

int excludedId;
vector< int > stack;

bool find_cycle( const int id )
{
	vertex& v = verts[ id ];
	if ( v.visitedId == excludedId )
	{
		if ( excludedId < 0 && v.isOnStack )
		{
			int curr = stack.size() - 1;
			while ( 1 )
			{
				verts[ stack[ curr ] ].onCycleId = excludedId;
				if ( stack[ curr ] == id )
				{
					break;
				}
				--curr;
			}

			const int n = verts.size();
			for ( int i = 0; i < n; ++i )
			{
				if ( verts[ i ].onCycleId != excludedId )
				{
					verts[ i ].isInResult = false;
				}
			}
		}

		return v.isOnStack;
	}

	v.visitedId = excludedId;
	v.isOnStack = true;
	if ( excludedId < 0 )
	{
		stack.push_back( id );
	}
	for ( set< int >::iterator it = v.links.begin(); it != v.links.end(); ++it )
	{
		const int linkId = *it;
		const vertex& link = verts[ linkId ];
		if ( linkId == excludedId )
		{
			continue;
		}
		if ( find_cycle( linkId ) )
		{
			v.isOnStack = false;
			if ( excludedId < 0 )
			{
				stack.pop_back();
			}
			return true;
		}
	}
	v.isOnStack = false;
	if ( excludedId < 0 )
	{
		stack.pop_back();
	}
	return false;
}

int main()
{
	int n, m;
	cin >> n >> m;

	verts.resize( n );
	for ( int i = 0; i < m; ++i )
	{
		int a, b;
		cin >> a >> b;
		--a;
		--b;
		verts[ a ].links.insert( b );
	}

	// Find any cycle

	const int steps = 10;
	for ( int i = 0; i < steps; ++i )
	{
		excludedId = -( i + 1 );
		if ( !find_cycle( i * n / steps ) )
		{
			cout << "NIE";
			return 0;
		}
	}

	// Brute-force

	for ( int i = 0; i < n; ++i )
	{
		if ( !verts[ i ].isInResult )
		{
			continue;
		}

		excludedId = i;
		for ( int j = 0; j < n; ++j )
		{
			if ( i != j && find_cycle( j ) )
			{
				verts[ i ].isInResult = false;
				break;
			}
		}
	}

	int count = 0;
	for ( int i = 0; i < n; ++i )
	{
		if ( verts[ i ].isInResult )
		{
			++count;
		}
	}
	cout << count << endl;
	for ( int i = 0; i < n; ++i )
	{
		if ( verts[ i ].isInResult )
		{
			cout << ( i + 1 ) << " ";
		}
	}

	return 0;
}