#include <fstream>
#include <vector>

using namespace std;

const int N = 1e5 + 5;

vector<int> gr[N];
vector<pair<int, int>> ans;
bool viz1[N], viz2[N];

void dfs1(int nod, int tata) {
  viz1[nod] = true;
  for (auto vec : gr[nod])
    if (!viz1[vec])
      dfs1(vec, nod);
  if (tata != 0)
    ans.emplace_back(nod, tata);
}

void dfs2(int nod) {
  viz2[nod] = true;
  for (auto vec : gr[nod])
    if (!viz2[vec]) {
      ans.emplace_back(nod, vec);
      dfs2(vec);
    }
}

int main() {
  ifstream cin("mesaj4.in");
  ofstream cout("mesaj4.out");
  int n, m;
  cin >> n >> m;
  while (m--) {
    int x, y;
    cin >> x >> y;
    gr[x].push_back(y);
    gr[y].push_back(x);
  }
  cin.close();
  dfs1(1, 0);
  for (int i = 1; i <= n; ++i)
    if (!viz1[i]) {
      cout << "-1\n";
      cout.close();
      return 0;
    }
  dfs2(1);
  cout << ans.size() << "\n";
  for (auto i : ans)
    cout << i.first << " " << i.second << "\n";
  cout.close();
  return 0;
}