using namespace std;
#include <fstream>
#include <cstdio>
#include <vector>
#include <utility>
#define MAX_N 100005
#define pb push_back
int n, m, v[MAX_N];
vector<int> G[MAX_N];
vector< pair<int, int> > rez;

void dfs(int x)
{
    v[x] = 1;
    for(int i=0; i<G[x].size(); ++i)
        if(v[G[x][i]]==0)
        {
            rez.pb( make_pair(x, G[x][i]));
            dfs(G[x][i]);
        }
}

int main()
{
    int x, y, i;
    freopen("mesaj4.out", "w", stdout);
    ifstream fin("mesaj4.in");

    fin>>n>>m;
    for(i=1; i<=m; i++)
    {
        fin>>x>>y;
        G[x].pb(y);
        G[y].pb(x);
    }
    fin.close();

    dfs(1);
    int pp=1;
    for(i=1; i<=n && pp; ++i)
        if(!v[i])
            pp=0;
    if(pp)
    {
        printf("%d\n", n*2-2);
        for(i=rez.size()-1; i>=0; i--)
            printf("%d %d\n", rez[i].second, rez[i].first);
        for(i=0; i<rez.size(); i++)
            printf("%d %d\n", rez[i].first, rez[i].second);
    }
    else
        printf("-1");
    return 0;
}