#include <bits/stdc++.h>

using namespace std;

ifstream fin("mesaj4.in");
ofstream fout("mesaj4.out");

int n, m, x, y, ss;
bool viz[100001];
vector<int>G[100001];
pair<int, int>sol[100001];

void dfs(int nod)
{
    viz[nod]=1;
    for(auto it:G[nod])
    {
        if(!viz[it])
        {
            dfs(it);
            sol[++ss].first=it;
            sol[ss].second=nod;
        }
    }
}

int main()
{
    fin>>n>>m;
    for(int i=1; i<=m; ++i)
    {
        fin>>x>>y;
        G[x].push_back(y);
        G[y].push_back(x);
    }
    dfs(1);
    bool ok=1;
    for(int i=1; i<=n && ok; ++i) if(!viz[i]) ok=0;
    if(!ok)
    {
        fout<<"-1\n";
        return 0;
    }
    fout<<2*ss<<"\n";
    for(int i=1; i<=ss; ++i) fout<<sol[i].first<<" "<<sol[i].second<<"\n";
    for(int i=ss; i>=1; --i) fout<<sol[i].second<<" "<<sol[i].first<<"\n";
    return 0;
}