#include<bits/stdc++.h>
using namespace std;
ifstream f("mesaj4.in");
ofstream g("mesaj4.out");
int i,n,m,x,y;
bool been[1<<17];
vector<int> E[1<<17];
vector <pair <int,int> > ans;
void dfs(int x)
{
    been[x]=1;
    for(vector<int>::iterator it=E[x].begin();it!=E[x].end();++it)
        if(!been[*it])
        {
            ans.push_back(make_pair(*it,x));
            dfs(*it);
        }
}
int main()
{
    f>>n>>m;
    for(i=1;i<=m;++i)
    {
        f>>x>>y;
        E[x].push_back(y);
        E[y].push_back(x);
    }
    dfs(1);
    if(ans.size()<n-1)
    {
        g<<"-1\n";
        return 0;
    }
    g<<n*2-2<<'\n';
    for(i=n-2;i>=0;--i)
        g<<ans[i].first<<' '<<ans[i].second<<'\n';
    for(i=0;i<n-1;++i)
        g<<ans[i].second<<' '<<ans[i].first<<'\n';
    return 0;
}