#include <bits/stdc++.h>
#define Mmax 500005
#define Nmax 100005
using namespace std;

ifstream f("ciclueuler.in");
ofstream g("ciclueuler.out");

struct three{
    int first,second;
    bool third;
};
three M[Mmax];
vector<int>G[Nmax],sol;
int n,m;

bool Verif()
{
    for(int i = 1; i <= n; i++)
        if(G[i].size() & 1)
        return 0;
    return 1;
}

void Euler(int nod)
{

    stack<int> S;
    S.push(1);
    while(!S.empty())
    {
        int nod = S.top();
        if(!G[nod].size())
        {
            sol.push_back(nod);
            S.pop();
        }
        else
        {
            int muchie = G[nod].back();
            G[nod].pop_back();
            if(!M[muchie].third)
            {
                M[muchie].third = true;
                if(M[muchie].first == nod)
                    S.push(M[muchie].second);
                else
                    S.push(M[muchie].first);
            }
        }
    }


}


int main()
{
    f>>n>>m;
    for(int i = 1; i <= m; i++)
    {
        int x,y;
        f>>x>>y;
        M[i]={x,y,false};
        G[x].push_back(i);
        G[y].push_back(i);

    }
    if(!Verif())
    {
        g<<"-1\n";
        return 0;
    }

    Euler(1);
g<<sol.size()-1<<"\n";
for(vector<int>::iterator it = sol.begin(); it < sol.end()-1; it++)
    g<<*it<<" ";


    return 0;
}