#include <bits/stdc++.h>
using namespace std;
const int Max = 1e5 + 10;
int h[Max];
vector<int> A;
vector<int> N[Max];
bool seen[Max];
int p[Max];
int ans;
int sttime[Max];
int ttime;
int DFS(int v = 1) {
  sttime[v] = ttime++;
  seen[v] = true;
  int ans = 100000000;
  for (int u : N[v])
    if (!seen[u]) {
      p[u] = v;
      h[u] = h[v] + 1;
      ans = min(ans, DFS(u));
    } else if (h[u] != h[v] - 1)
      ans = min(ans, h[u]);
  if (ans >= h[v] && v != 1) A.push_back(v);
  return ans;
}
int main() {
  int n, m;
  cin >> n >> m;
  for (int i = 0; i < m; i++) {
    int a, b;
    cin >> a >> b;
    N[a].push_back(b);
    N[b].push_back(a);
  }
  if (n == 2) {
    cout << -1 << '\n';
    return 0;
  }
  DFS();
  if (A.size() == 0) {
    cout << 0 << '\n';
    return 0;
  }
  vector<int> IMP;
  IMP.push_back(A[0]);
  for (int i = 1; i < A.size(); i++)
    if (sttime[A[i]] > sttime[A[i - 1]]) IMP.push_back(A[i]);
  if (sttime[A.back()] < sttime[A[0]]) IMP.push_back(p[A.back()]);
  if (IMP.size() == 1) {
    cout << 1 << '\n';
    int v = IMP[0], u = p[IMP[0]];
    if (N[v].size() >= 2)
      if (N[v][0] == u)
        cout << N[v][1] << " " << u << '\n';
      else
        cout << N[v][0] << " " << u << '\n';
    else
      cout << v << " " << p[u] << '\n';
    return 0;
  }
  vector<pair<int, int> > ANS;
  for (int i = 0; i < IMP.size() / 2; i++)
    ANS.push_back(make_pair(IMP[i], IMP[i + IMP.size() / 2]));
  if (IMP.size() & 1) ANS.push_back(make_pair(IMP[0], IMP.back()));
  cout << ANS.size() << '\n';
  for (pair<int, int> x : ANS) cout << x.first << " " << x.second << '\n';
}
