#include <bits/stdc++.h>
using namespace std;
const int mn = 1e6 + 10;
const long long mod = 998244353;
vector<int> g[mn];
int cc[mn], down[mn], down2[mn], up[mn];
int ct = 0;
void dfs1(int x, int p) {
  cc[x] = ct;
  for (int y : g[x])
    if (y != p) {
      dfs1(y, x);
      if (down[y] + 1 > down[x])
        down2[x] = down[x], down[x] = down[y] + 1;
      else if (down[y] + 1 > down2[x])
        down2[x] = down2[x] = down[y] + 1;
    }
}
void dfs2(int x, int p) {
  for (int y : g[x])
    if (y != p) {
      up[y] = up[x] + 1;
      if (down[y] + 1 == down[x])
        up[y] = max(up[y], down2[x] + 1);
      else
        up[y] = max(up[y], down[x] + 1);
      dfs2(y, x);
    }
}
int mo;
int mi[mn], pos[mn];
int main() {
  int n, m;
  scanf("%d%d", &n, &m);
  for (int i = 0; i < m; i++) {
    int a, b;
    scanf("%d%d", &a, &b);
    g[a].push_back(b);
    g[b].push_back(a);
  }
  for (int i = 1; i <= n; i++)
    if (!cc[i]) ++ct, dfs1(i, 0), dfs2(i, 0);
  for (int i = 1; i <= n; i++) mo = max(mo, max(down[i], up[i]));
  memset(mi, 0x3f, sizeof(mi));
  for (int i = 1; i <= n; i++) {
    if (max(down[i], up[i]) < mi[cc[i]])
      mi[cc[i]] = max(down[i], up[i]), pos[cc[i]] = i;
  }
  vector<int> o(ct);
  iota(o.begin(), o.end(), 1);
  sort(o.begin(), o.end(), [](int a, int b) { return mi[a] > mi[b]; });
  if (o.size() == 1)
    printf("%d\n", mo);
  else if (o.size() == 2)
    printf("%d\n", max(mo, mi[o[0]] + mi[o[1]] + 1));
  else
    printf("%d\n",
           max(max(mo, mi[o[0]] + mi[o[1]] + 1), mi[o[1]] + mi[o[2]] + 2));
  for (int i = 1; i < o.size(); i++) {
    printf("%d %d\n", pos[o[i]], pos[o[0]]);
  }
}
