#include <iostream>
#include <fstream>
#include <vector>
#define N 100105
#define M 500105
using namespace std;
ifstream fin ("ciclueuler.in");
ofstream fout ("ciclueuler.out");
int n,m,viz[M];
vector <pair<int,int> > a[N];
int q[M],sol[M],k;
void dfs(int i)
{
    int j,v;
    while (!a[i].empty())
    {
        j=a[i].back().second;
        v=a[i].back().first;
        a[i].pop_back();
        if (!viz[j])
        {
            viz[j]=1;
            dfs(v);
        }
    }
    k++;
    sol[k]=i;
}
int main()
{
    int x,y,i;
    fin>>n>>m;
    for (i=1;i<=m;i++)
    {
        fin>>x>>y;
        a[x].push_back(make_pair(y,i));
        a[y].push_back(make_pair(x,i));
    }
    for (i=1;i<=n;i++)
        if (a[i].size()%2==1)
        {
            fout<<"-1"<<"\n";
            return 0;
        }
    dfs(1);
    fout<<m<<"\n";
   for (i=1; i<k; i++)
        fout<<sol[i]<<" ";
    fin.close();
    fout.close();
    return 0;
}