#include <bits/stdc++.h>
 using namespace std;
 ifstream f("ciclueuler.in");
 ofstream g("ciclueuler.out");
 int n, m , a[202][202] , x , y , q[40001] , cnt;
void Euler(int k)
{
    for(int i = 1 ; i <= n ; i ++)
        if(a[k][i])
        {
            a[k][i] = a[i][k] = 0;
            Euler(i);
        }
    q[++cnt] = k;
}
int main()
{
    f>> n>>m;
    while(f >> x >> y)
        a[x][y] = a[y][x] = 1;
    Euler(1);
      if (cnt%2==1) g<<-1;
    else {
    g<< cnt << '\n';
    for(int i = 1 ; i <= cnt ; i++)
        g<< q[i] << " ";
    }
}