#include <bits/stdc++.h>
using namespace std;
const int MN = 900 + 90;
int h[MN];
int par[MN];
bool mark[MN];
vector<int> adj[MN];
vector<pair<int, int>> edges;
vector<int> adj2[MN];
vector<int> leaves;
bool mat[MN][MN];
int root(int v) { return par[v] == v ? v : par[v] = root(par[v]); }
void merg(int u, int v) { par[root(v)] = root(u); }
int dfs(int v, int p) {
  mark[v] = true;
  int mx = h[v];
  for (int i : adj[v]) {
    if (!mark[i]) {
      h[i] = h[v] + 1;
      mx = min(mx, dfs(i, v));
    } else if (i != p) {
      mx = min(mx, h[i]);
    }
  }
  if (mx < h[v]) merg(v, p);
  return mx;
}
void dfs2(int v, int p) {
  if (adj2[v].size() == 1) leaves.push_back(v);
  for (int i : adj2[v]) {
    if (i != p) dfs2(i, v);
  }
}
void print_edge(int u, int v) {
  if (!mat[u][v]) {
    cout << u + 1 << " " << v + 1 << endl;
    return;
  }
  for (int i = 0; i < 1000; i++) {
    if (root(i) == u && !mat[i][v]) {
      cout << i + 1 << " " << v + 1 << endl;
      return;
    } else if (root(i) == v && !mat[u][i]) {
      cout << i + 1 << " " << u + 1 << endl;
      return;
    }
  }
}
int main() {
  ios::sync_with_stdio(0);
  int n, m;
  cin >> n >> m;
  if (n == 2 && m == 1) {
    cout << -1;
    return 0;
  }
  for (int i = 0; i < m; i++) {
    int u, v;
    cin >> u >> v;
    u--;
    v--;
    adj[u].push_back(v);
    adj[v].push_back(u);
    edges.push_back(make_pair(u, v));
    mat[u][v] = mat[v][u] = 1;
  }
  iota(par, par + n, 0);
  dfs(0, 0);
  for (int i = 0; i < m; i++) {
    int x = root(edges[i].first), y = root(edges[i].second);
    if (x != y) {
      adj2[x].push_back(y);
      adj2[y].push_back(x);
    }
  }
  dfs2(root(0), -1);
  cout << (leaves.size() + 1) / 2 << endl;
  for (int i = 0; i < int(leaves.size()) / 2; i++) {
    print_edge(leaves[i], leaves[i + int(leaves.size()) / 2]);
  }
  if (leaves.size() % 2) {
    print_edge(leaves[0], leaves[leaves.size() - 1]);
  }
}
