#include <bits/stdc++.h>
using namespace std;
template <typename T>
void out(T x) {
  cout << x << endl;
  exit(0);
}
const int maxn = 1005;
int n, m, viz[maxn];
vector<int> g[maxn];
int maxd1, maxv1;
int maxd2, midv2, path[maxn];
void dfs1(int at, int p = -1, int d = 0) {
  viz[at] = 1;
  if (d > maxd1) {
    maxd1 = d;
    maxv1 = at;
  }
  for (auto to : g[at]) {
    if (p == to || viz[to]) continue;
    dfs1(to, at, d + 1);
  }
}
void dfs2(int at, int p = -1, int d = 0) {
  path[d] = at;
  if (d > maxd2) {
    maxd2 = d;
    midv2 = path[d / 2];
  }
  for (auto to : g[at]) {
    if (to == p) continue;
    dfs2(to, at, d + 1);
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> m;
  for (int i = 0; i < m; i++) {
    int u, v;
    cin >> u >> v;
    --u;
    --v;
    g[u].push_back(v);
    g[v].push_back(u);
  }
  vector<pair<int, int>> cc;
  for (int i = 0; i < n; i++) {
    if (!viz[i]) {
      maxd1 = maxv1 = -1;
      dfs1(i);
      maxd2 = midv2 = -1;
      dfs2(maxv1);
      cc.push_back({maxd2, midv2});
    }
  }
  sort(cc.begin(), cc.end(), greater<pair<int, int>>());
  if (cc.size() == 1) {
    out(cc.front().first);
  }
  int diam = cc[0].first;
  if (cc.size() > 1)
    diam = max(diam, (cc[0].first + 1) / 2 + (cc[1].first + 1) / 2 + 1);
  if (cc.size() > 2)
    diam = max(diam, (cc[2].first + 1) / 2 + (cc[1].first + 1) / 2 + 2);
  cout << diam << endl;
  for (int i = 1; i < (int)cc.size(); i++) {
    cout << cc[0].second + 1 << " " << cc[i].second + 1 << endl;
  }
  return 0;
}
