#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 2e5 + 5;
const int M = 1e6;
const long long INF = (long long)(1e18);
const int inf = 1e9;
const long long MOD = 1000000007LL;
int n, m;
vector<int> G[1005];
vector<pair<int, int> > newEdges;
vector<pair<int, int> > trees;
bool vis[1005];
vector<int> curTree;
int dist[1005];
void reset() {
  for (int i = 1; i <= n; i++) {
    dist[i] = 0;
  }
}
void dfs(int v, int p) {
  vis[v] = true;
  curTree.push_back(v);
  for (int to : G[v]) {
    if (to == p) continue;
    dfs(to, v);
  }
}
int dfs2(int v, int p) {
  if (p != -1) {
    dist[v] = dist[p] + 1;
  }
  int res = dist[v];
  for (int to : G[v]) {
    if (to == p) continue;
    res = max(res, dfs2(to, v));
  }
  return res;
}
pair<int, int> diam(int v, int p) {
  if (p != -1) {
    dist[v] = dist[p] + 1;
  }
  pair<int, int> res = make_pair(dist[v], v);
  for (int to : G[v]) {
    if (to == p) continue;
    pair<int, int> xd = diam(to, v);
    if (xd.first > res.first) {
      res = xd;
    }
  }
  return res;
}
int calcDiam(int v) {
  reset();
  pair<int, int> p = diam(v, -1);
  reset();
  pair<int, int> d = diam(p.second, -1);
  return d.first;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> n >> m;
  for (int i = 0; i < m; i++) {
    int u, v;
    cin >> u >> v;
    G[u].push_back(v);
    G[v].push_back(u);
  }
  for (int i = 1; i <= n; i++) {
    if (vis[i]) continue;
    curTree.clear();
    dfs(i, -1);
    int mini = inf;
    int chosen = -1;
    for (int v : curTree) {
      reset();
      int tmp = dfs2(v, -1);
      if (tmp < mini) {
        mini = tmp;
        chosen = v;
      }
    }
    int d = calcDiam(chosen);
    trees.emplace_back(d, chosen);
  }
  sort(trees.rbegin(), trees.rend());
  for (int i = 1; i < (int)trees.size(); i++) {
    int u = trees[0].second;
    int v = trees[i].second;
    newEdges.emplace_back(u, v);
    G[u].push_back(v);
    G[v].push_back(u);
  }
  cout << calcDiam(1) << "\n";
  for (pair<int, int> p : newEdges) {
    cout << p.first << " " << p.second << "\n";
  }
  return 0;
}
