#include <bits/stdc++.h>
using namespace std;
const int maxN = 1010;
vector<int> adj[maxN], vc;
int h[maxN], par[maxN];
vector<pair<int, int> > ans;
bool mark[maxN];
void dfs(int v, int p = -1) {
  if (p == -1) h[v] = 0;
  mark[v] = true;
  vc.push_back(v);
  par[v] = p;
  for (auto u : adj[v]) {
    if (u == p) continue;
    h[u] = h[v] + 1;
    dfs(u, v);
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n, m;
  cin >> n >> m;
  for (int i = 0; i < m; i++) {
    int u, v;
    cin >> u >> v;
    u--;
    v--;
    adj[u].push_back(v);
    adj[v].push_back(u);
  }
  int result = 0;
  for (int i = 0; i < n; i++) {
    if (mark[i]) continue;
    vc.clear();
    dfs(i, -1);
    int best = -1, maxi = -1;
    for (auto u : vc)
      if (h[u] > maxi) maxi = h[u], best = u;
    vc.clear();
    dfs(best, -1);
    best = -1, maxi = -1;
    for (auto u : vc)
      if (h[u] > maxi) maxi = h[u], best = u;
    result = max(result, h[best]);
    int len = h[best], curr = best;
    for (int i = 0; i < len / 2; i++) curr = par[curr];
    ans.push_back({(len + 1) / 2, curr});
  }
  sort(ans.begin(), ans.end());
  int sz = ans.size();
  if (sz >= 2) result = max(result, 1 + ans[sz - 1].first + ans[sz - 2].first);
  if (sz >= 3) result = max(result, 2 + ans[sz - 2].first + ans[sz - 3].first);
  cout << result << endl;
  for (int i = 0; i < sz - 1; i++)
    cout << ans[i].second + 1 << ' ' << ans[sz - 1].second + 1 << '\n';
}
