#include <bits/stdc++.h>
using namespace std;
int n, m;
vector<int> v[1005];
int ma, src;
bool vis[1005];
int p[1005];
void dfs(int s, int pa, int d) {
  p[s] = pa;
  vis[s] = 1;
  if (d >= ma) {
    src = s;
    ma = d;
  }
  for (int i = 0; i < v[s].size(); i++) {
    int u = v[s][i];
    if (u != pa) {
      dfs(u, s, d + 1);
    }
  }
}
int main() {
  int u = 0;
  ios::sync_with_stdio(0);
  cin >> n >> m;
  for (int i = 0; i < m; i++) {
    int x, y;
    cin >> x >> y;
    v[x].push_back(y);
    v[y].push_back(x);
  }
  int trees = 0, ans = 0;
  vector<int> vvv;
  vector<pair<int, int> > vv;
  for (int i = 1; i <= n; i++) {
    if (!vis[i]) {
      ma = 0;
      src = i;
      dfs(i, -1, 0);
      dfs(src, -1, 0);
      int node = src;
      for (int j = 0; j < ma / 2; j++) {
        node = p[node];
      }
      vv.push_back({ma, node});
      trees++;
      u = max(u, ma);
      vvv.push_back((ma + 1) / 2);
    }
  }
  sort(vvv.begin(), vvv.end());
  sort(vv.rbegin(), vv.rend());
  for (int i = vvv.size() - 1; i >= max(0, (int)(vvv.size() - 2)); i--) {
    ans += vvv[i];
  }
  int mi = 2;
  if (vvv.size() >= 3) {
    if (vvv[vvv.size() - 3] < vvv[vvv.size() - 1]) {
      mi = 1;
    }
  }
  ans += min(trees - 1, mi);
  ans = max(ans, u);
  cout << ans << endl;
  for (int i = 0; i < vv.size() - 1; i++) {
    cout << vv[0].second << " " << vv[i + 1].second << endl;
  }
  return 0;
}
