#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
/*
ID: samikgo1
TASK: wormhole
LANG: C++
*/
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll,ll> pll;
#define all(x) x.begin(),x.end()
#define allr(x) x.rbegin(),x.rend()
//#define f first
//#define s second
//#define x first
//#define y second
const int INF = 1e9;
const ll modulo = 1e9 + 7;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
//using namespace __gnu_pbds;
//#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define int ll
vector<int> adj[500001];
bool visited[500001];
bool colour[500001];
int c;
void dfs(int x, bool col){
if(visited[x] and colour[x]!=col){
cout<<"-1\n";exit(0);
}
if(visited[x])return;
if(col)c++;
colour[x]=col;
visited[x]=true;
for(int&i:adj[x])dfs(i,!col);
}
void solve(){
int n,m;
cin >> n >> m;
for (int i = 0; i < m; i++) {
int a,b;cin>>a>>b;
adj[a].emplace_back(b);
adj[b].emplace_back(a);
}
dfs(1,true);
for(int i=1;i<=n;i++)if(!visited[i])dfs(i,true);
cout << c << '\n';
for(int i=1;i<=n;i++)if(colour[i])cout<<i<<' ';
cout << '\n';
}
int32_t main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
// freopen("in.txt","r",stdin);
// freopen("out.txt","w",stdout);
// int t;
// cin >> t;
// while(t--)
solve();
}
