#include <bits/stdc++.h>

using namespace std;

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

const int kN = 1e5;
vector<int> g[1 + kN];
bitset<1 + kN> vis;
vector<pair<int, int>> sol;

void dfs(int u) {
  vis[u] = true;
  for (int v : g[u]) {
    if (!vis[v]) {
      sol.emplace_back(u, v);
      dfs(v);
    }
  }
}

void TestCase() {
  int n, m;
  fin >> n >> m;
  for (int i = 1; i <= m; ++i) {
    int u, v;
    fin >> u >> v;
    g[u].emplace_back(v);
    g[v].emplace_back(u);
  }
  dfs(1);
  if ((int)sol.size() != n - 1) {
    fout << "-1\n";
    return;
  }
  fout << 2 * sol.size() << '\n';
  for (int i = sol.size() - 1; i >= 0; --i) {
    fout << sol[i].second << ' ' << sol[i].first << '\n';
  }
  for (auto it : sol) {
    fout << it.first << ' ' << it.second << '\n';
  }
}

int main() {
  int tests = 1;
  for (int tc = 1; tc <= tests; ++tc) {
    TestCase();
  }
  fin.close();
  fout.close();
  return 0;
}