#include<iostream>
#include<fstream>
#include<vector>
#include<cstring>
#include<map>
#include<algorithm>
#include<set>
#include<deque>
#include<queue>
using namespace std;
 
const int mx=1000;
 
ifstream in("party.in");
ofstream out("party.out");
 
vector<int> g[mx],gt[mx],q;
bool visited[mx],possible=true;
int n,m,result[mx],cnt[mx];
 
int node(int x){
	if(x<0){
		return n+abs(x);
	}
	return x;
}
 
int val(int node){
	if(node>n){
		return n-node;
	}
	return node;
}
 
 
void read(){
	in>>n>>m;
	int a,b,c;
	for(int i=0;i<m;i++){
		in>>a>>b>>c;
		if(c==3){
			a=-a;
			b=-b;
		}
		else if(c==1){
			b=-b;
		}
		else if(c==2){
			a=-a;
		}
		g[node(-a)].push_back(node(b));
		gt[node(b)].push_back(node(-a));
		g[node(-b)].push_back(node(a));
		gt[node(a)].push_back(node(-b));
	}
}
 
void dfs1(int x){
	visited[x]=true;
	for(int k:g[x]){
		if(!visited[k])
			dfs1(k);
	}
	q.push_back(x);
}
 
void dfs2(int x,int value,int comp){
	cnt[x]=comp;	
	visited[x]=true;
	result[x]=value;
	result[node(-val(x))]=1-value;
 
	for(int k:gt[x]){
		if(!visited[k]){
			dfs2(k,value,comp);
		}
	}
}
void preprocess(){
	for(int i=1;i<=2*n;i++)
		result[i]=-1;
}
 
void solve(){
 
	for(int i=1;i<=2*n;i++){
		if(!visited[i]){
			dfs1(i);
		}
	}
	for(int i=1;i<mx;i++)
		visited[i]=false;
 
	int count=1;
	for(auto iter=q.rbegin();iter!=q.rend();iter++){
		if(!visited[*iter]){
			dfs2(*iter,1,count);
			count++;
		}
	} 
 
	for(int i=1;i<=n;i++){
		if(cnt[i]==cnt[n+i]){
			out<<-1;
			return;
		}
	}
	vector<int> ans;
 
	for(int i=1;i<=n;i++)
		if(result[i])
			ans.push_back(i);
 
	out<<ans.size()<<endl;
	for(int k:ans)
		out<<k<<'\n';
}
 
int main(){
	read();
	solve();
	return 0;
}