#include <bits/stdc++.h>
using namespace std;
long long modulo = 1e9 + 7;
template <class X>
void printarr(X arr[], int n) {
  for (int i = 0; i < n; ++i) cout << arr[i] << " ";
  cout << endl;
}
long long fpow(long long a, long long n) {
  long long ans = 1;
  while (n) {
    if (n & 1) ans = (ans * a) % modulo;
    a = (a * a) % modulo;
    n = n >> 1;
  }
  return ans;
}
const int M = 2e3 + 10;
vector<int> graph[M];
int visited[M] = {0};
int dfs(int source, int original) {
  visited[source] = original;
  int max_height = -1;
  for (int a : graph[source]) {
    if (visited[a] == original) continue;
    max_height = max(dfs(a, original), max_height);
  }
  return 1 + max_height;
}
void solve(int source, int distance) {
  visited[source] = distance;
  for (int a : graph[source]) {
    if (visited[a] == -1) solve(a, distance + 1);
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int n, m;
  cin >> n >> m;
  int parent[n + 1], min_distance[n + 1];
  for (int i = 0; i <= n; ++i) {
    parent[i] = i;
    min_distance[i] = INT_MAX;
  }
  for (int i = 0; i < m; ++i) {
    int u, v;
    cin >> u >> v;
    graph[u].push_back(v);
    graph[v].push_back(u);
  }
  if (n == 1) {
    cout << 0 << endl;
    return 0;
  }
  for (int i = 1; i <= n; ++i) {
    int distance = dfs(i, i);
    if (distance < min_distance[i]) {
      for (int j = 1; j <= n; ++j) {
        if (visited[j] == i) {
          min_distance[j] = distance;
          parent[j] = i;
        }
      }
    }
  }
  vector<pair<int, int> > connected_components;
  for (int i = 1; i <= n; ++i) {
    if (parent[i] == i) connected_components.push_back({min_distance[i], i});
  }
  sort(connected_components.begin(), connected_components.end(),
       greater<pair<int, int> >());
  int cnt = connected_components.size();
  for (int i = 1; i < cnt; ++i) {
    int u = connected_components[0].second, v = connected_components[i].second;
    graph[u].push_back(v);
    graph[v].push_back(u);
  }
  for (int i = 1; i <= n; ++i) visited[i] = -1;
  solve(connected_components[0].second, 0);
  int distance = 0, leaf;
  for (int i = 1; i <= n; ++i) {
    if (visited[i] > distance) {
      leaf = i;
      distance = visited[i];
    }
  }
  for (int i = 1; i <= n; ++i) visited[i] = 0;
  cout << dfs(leaf, leaf) << endl;
  for (int i = 1; i < cnt; ++i)
    cout << connected_components[0].second << " "
         << connected_components[i].second << endl;
  return 0;
}
