#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
const int N = 200031;
int n, m;
vector<int> g[N];
int used[N];
vector<int> cur_comp;
void dfs(int v) {
  used[v] = 1;
  cur_comp.push_back(v);
  for (int i = 0; i < g[v].size(); i++) {
    int to = g[v][i];
    if (used[to]) continue;
    dfs(to);
  }
}
vector<pair<int, vector<int> > > comps;
vector<pair<int, int> > ans;
int answ;
int get_mid(vector<int> v) { return v[v.size() / 2]; }
int par[N];
void trace(int v) {
  if (used[v] == 0) used[v] = 1;
  for (int i = 0; i < g[v].size(); i++) {
    int to = g[v][i];
    if (used[to]) continue;
    used[to] = used[v] + 1;
    par[to] = v;
    trace(to);
  }
}
vector<int> get_diam() {
  for (int i = 0; i < cur_comp.size(); i++) {
    int id = cur_comp[i];
    used[id] = 0;
  }
  trace(cur_comp[0]);
  int root = cur_comp[0];
  for (int i = 0; i < cur_comp.size(); i++) {
    if (used[cur_comp[i]] > used[root]) root = cur_comp[i];
  }
  for (int i = 0; i < cur_comp.size(); i++) {
    used[cur_comp[i]] = 0;
  }
  trace(root);
  int root2 = cur_comp[0];
  for (int i = 0; i < cur_comp.size(); i++) {
    if (used[cur_comp[i]] > used[root2]) root2 = cur_comp[i];
  }
  vector<int> ret;
  while (root2 != root) {
    ret.push_back(root2);
    root2 = par[root2];
  }
  ret.push_back(root);
  return ret;
}
int main() {
  cin >> n >> m;
  for (int i = 1; i <= m; i++) {
    int a, b;
    cin >> a >> b;
    g[a].push_back(b);
    g[b].push_back(a);
  }
  while (true) {
    comps.clear();
    for (int i = 1; i <= n; i++) {
      used[i] = 0;
    }
    for (int i = 1; i <= n; i++) {
      if (used[i]) continue;
      cur_comp.clear();
      dfs(i);
      comps.push_back(make_pair(get_diam().size() - 1, get_diam()));
    }
    sort(comps.begin(), comps.end());
    answ = comps[0].first;
    if (comps.size() == 1) break;
    ans.push_back(make_pair(get_mid(comps[comps.size() - 1].second),
                            get_mid(comps[comps.size() - 2].second)));
    pair<int, int> new_edge = ans.back();
    g[new_edge.first].push_back(new_edge.second);
    g[new_edge.second].push_back(new_edge.first);
  }
  cout << answ << endl;
  for (int i = 0; i < ans.size(); i++) {
    cout << ans[i].first << " " << ans[i].second << endl;
  }
  return 0;
}
