#include <bits/stdc++.h>
using namespace std;
vector<vector<int>> g;
vector<pair<int, int>> d;
vector<int> used;
pair<int, int> dfs(int v, int p) {
  used[v] = 1;
  pair<int, int> cnt = {0, v};
  for (auto u : g[v]) {
    if (u == p) continue;
    pair<int, int> now = dfs(u, v);
    if (now.first >= cnt.first) {
      cnt = now;
    }
  }
  cnt.first++;
  return cnt;
}
int ans = 0, md = 0;
int get(int v, int p, int l, int d) {
  if (v == l) return 0;
  if (g[v].size() == 1 && v != p) return -1e9;
  for (auto u : g[v]) {
    if (u == p) continue;
    int cnt = get(u, v, l, d);
    if (cnt >= 0) {
      if (cnt == d) {
        ans = u;
      }
      return cnt + 1;
    }
  }
}
pair<int, int> find(int v) {
  if (!g[v].size()) return {0, v};
  pair<int, int> f = dfs(v, v);
  pair<int, int> s = dfs(f.second, f.second);
  pair<int, int> way = {f.second, s.second};
  int diam = s.first - 1;
  md = max(md, diam);
  ans = way.first;
  get(way.first, way.first, way.second, diam / 2);
  return {(diam + 1) / 2, ans};
}
int main() {
  int n, m;
  cin >> n >> m;
  g.resize(n);
  for (int i = 0; i < m; i++) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    g[x].push_back(y);
    g[y].push_back(x);
  }
  used.resize(n);
  vector<pair<int, int>> diams;
  for (int i = 0; i < n; i++) {
    if (!used[i]) {
      diams.push_back(find(i));
    }
  }
  sort(diams.rbegin(), diams.rend());
  if (diams.size() == 1) {
    cout << md;
    return 0;
  }
  md = max(md, diams[0].first + diams[1].first + 1);
  if (diams.size() >= 3) {
    md = max(md, diams[1].first + diams[2].first + 2);
  }
  cout << md << endl;
  for (int i = 1; i < diams.size(); i++) {
    cout << diams[i].second + 1 << " " << diams[0].second + 1 << endl;
  }
}
