#include <bits/stdc++.h>
using namespace std;
ifstream f("ciclueuler.in");
ofstream g("ciclueuler.out");
const int NMAX = 100005, MMAX = 500005;
vector<int> G[NMAX];
bool vizitat[MMAX];
int from[MMAX], to[MMAX],nr_vec[NMAX];
int euler(int);
int main() {

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

    for (int i = 1; i <= n; ++i)
     {
        if (nr_vec[i]%2==1)
            {
            g << "-1\n";
            return 0;
           }
    }

euler(1);
return 0;

}

int euler (int v)
  {  while(size(G[v])!=0)
        {w = auto it:G[v];
        vizitat[w]=0;
        sterge_muchie (v, w);
        euler (w);
        }
    //adauga v la ciclu
  }