#include <bits/stdc++.h>
using namespace std;
int n, m, rt, mx, cnt, ans;
int vis[1005], d[1005];
vector<int> g[1005];
struct node {
  int u, len;
  bool operator<(const node &t) const { return len > t.len; }
} a[1005];
void dfs(int u, int fa, int dep) {
  d[u] = dep;
  vis[u] = 1;
  if (d[u] > d[rt]) rt = u;
  for (int i = 0; i < g[u].size(); i++) {
    int v = g[u][i];
    if (v == fa) continue;
    dfs(v, u, dep + 1);
  }
}
void dfs1(int u, int fa, int dep) {
  d[u] = dep;
  mx = max(mx, dep);
  for (int i = 0; i < g[u].size(); i++) {
    int v = g[u][i];
    if (v == fa) continue;
    dfs1(v, u, dep + 1);
    d[u] = max(d[u], d[v]);
  }
  if (d[u] == mx && dep == mx / 2) rt = u;
}
int main() {
  ios::sync_with_stdio(false);
  cin >> n >> m;
  int x, y;
  for (int i = 1; i <= m; i++) {
    cin >> x >> y;
    g[x].push_back(y);
    g[y].push_back(x);
  }
  for (int i = 1; i <= n; i++) {
    if (!vis[i]) {
      rt = i;
      mx = 0;
      dfs(i, 0, 0);
      dfs1(rt, 0, 0);
      a[++cnt].u = rt;
      a[cnt].len = mx / 2 + mx % 2;
      ans = max(ans, mx);
    }
  }
  sort(a + 1, a + 1 + cnt);
  if (cnt > 1) ans = max(ans, a[1].len + a[2].len + 1);
  if (cnt > 2) ans = max(ans, a[2].len + a[3].len + 2);
  cout << ans << endl;
  for (int i = 2; i <= cnt; i++) {
    cout << a[1].u << " " << a[i].u << endl;
  }
}
