#include <bits/stdc++.h>
using namespace std;
int dirx[] = {-1, -1, -1, 0, 0, 1, 1, 1};
int diry[] = {-1, 0, 1, -1, 1, -1, 0, 1};
int n, m;
vector<int> g[1101], cur;
int distx[1002], disty[1002];
int vis[1002];
int bfs(int x, int dist[1002]) {
  queue<int> q;
  q.push(x);
  dist[x] = 0;
  int lst = -1;
  while (q.size()) {
    int u = q.front();
    lst = u;
    q.pop();
    for (auto v : g[u]) {
      if (dist[v] > dist[u] + 1) {
        dist[v] = dist[u] + 1;
        q.push(v);
      }
    }
  }
  return lst;
}
void dfs(int i) {
  vis[i] = 1;
  cur.push_back(i);
  for (auto j : g[i]) {
    if (!vis[j]) dfs(j);
  }
}
vector<pair<int, int> > cent;
int main() {
  cin >> n >> m;
  for (long long i = 0; i < m; i++) {
    int u, v;
    cin >> u >> v;
    g[u].push_back(v);
    g[v].push_back(u);
  }
  for (long long i = 0; i < n + 1; i++) distx[i] = disty[i] = 1e7;
  for (long long i = (1); i <= (n); ++i) {
    if (!vis[i]) {
      cur.clear();
      dfs(i);
      int x = bfs(i, distx);
      int y = bfs(x, disty);
      for (auto v : cur) {
        distx[v] = 1e7;
      }
      bfs(y, distx);
      int dis = disty[y];
      int cen;
      for (auto v : cur)
        if (distx[v] == dis / 2 && disty[v] == dis - dis / 2) cen = v;
      cent.push_back(make_pair(dis, cen));
    }
  }
  sort(cent.begin(), cent.end());
  vector<int> ans;
  int maxi = cent.back().first;
  ans.push_back((maxi + 1) / 2);
  ans.push_back(maxi - ans.back());
  int ever = cent.back().second;
  cent.pop_back();
  for (auto v : cent) {
    ans.push_back((v.first + 1) / 2 + 1);
  }
  sort(ans.begin(), ans.end());
  cout << ans.back() + ans[ans.size() - 2] << "\n";
  for (auto v : cent) cout << ever << " " << v.second << "\n";
}
