#include <bits/stdc++.h>
using namespace std;
const int N = 1e3 + 10;
int vis[N];
vector<int> G[N];
vector<int> li;
int ff, par[N];
pair<int, int> dfs(int fa, int u) {
  vis[u] = 1;
  pair<int, int> res = pair<int, int>(0, u), tmp = pair<int, int>(0, 0);
  for (int v : G[u]) {
    if (v != fa) {
      par[v] = u;
      pair<int, int> t = dfs(u, v);
      res = max(res, t);
      t.second = v;
      tmp = max(tmp, t);
    }
  }
  res.first++;
  return res;
}
int main() {
  int n, m;
  cin >> n >> m;
  for (int i = 0, u, v; i < m; i++) {
    cin >> u >> v;
    G[u].push_back(v);
    G[v].push_back(u);
  }
  vector<int> pt;
  int mxd = 0, mxi = 0;
  for (int i = 1; i <= n; i++) {
    if (vis[i]) continue;
    ff = 0;
    pair<int, int> st = dfs(-1, i);
    ff = 1;
    pair<int, int> en = dfs(-1, st.second);
    int now = en.second;
    while (now != st.second) {
      li.push_back(now);
      now = par[now];
    }
    li.push_back(st.second);
    pt.push_back(li[en.first / 2]);
    if (en.first > mxd) {
      mxd = en.first;
      mxi = pt.back();
    }
    li.clear();
  }
  for (int i = 0; i < pt.size(); i++) {
    if (mxi == pt[i]) continue;
    G[mxi].push_back(pt[i]);
    G[pt[i]].push_back(mxi);
  }
  cout << dfs(-1, dfs(-1, pt[0]).second).first - 1 << endl;
  for (int i = 0; i < pt.size(); i++) {
    if (mxi == pt[i]) continue;
    printf("%d %d\n", mxi, pt[i]);
  }
  return 0;
}
