#include <bits/stdc++.h>
using namespace std;
template <class T>
inline bool uin(T& a, T b) {
  return a > b ? (a = b, true) : false;
}
template <class T>
inline bool uax(T& a, T b) {
  return a < b ? (a = b, true) : false;
}
const int nax = 1010;
vector<int> edge[nax];
int N, M;
int done[nax], par[nax];
int furthest, max_dist;
void dfs(int p, int col, int dist = 0) {
  if (done[p] == col) return;
  done[p] = col;
  if (uax(max_dist, dist)) furthest = p;
  for (int i : edge[p]) {
    if (i == par[p]) continue;
    par[i] = p;
    dfs(i, col, dist + 1);
  }
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> N >> M;
  for (int u, v, i = 0; i < M; ++i) {
    cin >> u >> v;
    edge[u - 1].push_back(v - 1);
    edge[v - 1].push_back(u - 1);
  }
  int max_dia = 0;
  vector<pair<int, int>> centeroids;
  for (int i = 0; i < N; ++i) {
    if (done[i] == 2) continue;
    max_dist = -1;
    dfs(i, 1);
    max_dist = -1;
    par[furthest] = -1;
    dfs(furthest, 2);
    int c = furthest;
    for (int j = 0; j < max_dist / 2; ++j) c = par[c];
    uax(max_dia, max_dist);
    centeroids.emplace_back((max_dist + 1) / 2, c);
  }
  sort(centeroids.rbegin(), centeroids.rend());
  int ans = max_dia;
  if (centeroids.size() > 1)
    uax(ans, centeroids[0].first + centeroids[1].first + 1);
  if (centeroids.size() > 2)
    uax(ans, centeroids[1].first + centeroids[2].first + 2);
  cout << ans << '\n';
  for (int i = 1; i < (int)centeroids.size(); ++i)
    cout << centeroids[i].second + 1 << ' ' << centeroids[0].second + 1 << '\n';
}
