#include <iostream>
#include <fstream>
#include <vector>
#include <stack>

using namespace std;

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

int n, m;
bool use[100100];
vector<vector<int>> adj;
vector<pair<int, int>> ans;

void solve(int node)
{
    use[node] = true;

    for(auto it : adj[node])
    {
        if(!use[it])
        {
            ans.emplace_back(it, node);
            solve(it);
        }
    }
}

int main()
{
    in >> n >> m;

    adj.resize(n + 5);

    for(int i = 1; i <= m; i++)
    {
        int x, y;
        in >> x >> y;

        adj[x].push_back(y);
        adj[y].push_back(x);
    }

    solve(1);

    for(int i = 1; i <= n; i++)
    {
        if(!use[i])
        {
            out << -1 << '\n';
            return 0;
        }
    }

    int len = ans.size();

    out << 2 * len << '\n';

    for(int i = len - 1; i >= 0; i--)
        out << ans[i].first << ' ' << ans[i].second << '\n';

    for(int i = 0; i < len; i++)
        out << ans[i].second << ' ' << ans[i].first << '\n';

    return 0;
}