#include <fstream>
#include <vector>
#include <algorithm>
using namespace std;
ifstream cin("mesaj4.in");
ofstream cout("mesaj4.out");
const int nmax = 1e5+5;
vector<vector<int>> gf(nmax);
vector<pair<int,int>> sol;
int n, m, nr, x, y, viz[nmax];

void dfs(int tata)
{
    viz[tata]=1;
    for(auto nod : gf[tata])
    {
        if(!viz[nod])
        {
            dfs(nod);
            sol.push_back({nod,tata});
        }
    }
}

int main()
{
    cin>>n>>m;
    for(int i=1; i<=m; i++)
    {
        cin>>x>>y;
        gf[x].push_back(y);
        gf[y].push_back(x);
    }

    for(int i=1; i<=n; i++)
    {
        if(!viz[i])
        {
            nr++;
            dfs(i);
        }
    }

    if(nr!=1)
    {
        cout<<-1;
        exit(0);
    }
    else
        cout<<2*n-2<<endl;

    for(auto solcrt : sol)
        cout<<solcrt.first<<' '<<solcrt.second<<'\n';
    reverse(sol.begin(),sol.end());
    for(auto solcrt : sol)
        cout<<solcrt.second<<' '<<solcrt.first<<'\n';
    return 0;
}