#include <bits/stdc++.h>
#pragma comment(linker, "/stack:225450978")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
const long long Mod = 1000000007LL, INF = 1e9, LINF = 1e18;
const long double Pi = 3.141592653589793116, EPS = 1e-9,
                  Gold = ((1 + sqrt(5)) / 2);
long long keymod[] = {1000000007LL, 1000000009LL, 1000000021LL, 1000000033LL};
long long keyCount = sizeof(keymod) / sizeof(long long);
template <class T>
int getbit(T s, int i) {
  return (s >> i) & 1;
}
template <class T>
T onbit(T s, int i) {
  return s | (T(1) << i);
}
template <class T>
T offbit(T s, int i) {
  return s & (~(T(1) << i));
}
template <class T>
int cntbit(T s) {
  return __builtin_popcountll(s);
}
auto TimeStart = chrono::steady_clock::now();
auto TimeEnd = chrono::steady_clock::now();
void ControlIO(int argc, char* argv[]);
void TimerStart();
void TimerStop();
void Exit();
long long n, x;
vector<bool> vis(1000003, false);
set<long long> S;
void Input() {
  cin >> n;
  while (n--) {
    cin >> x;
    vis[x] = true;
  }
}
void Solve() {
  long long symmetry = 0;
  for (long long i = 1; i <= 500000; i++) {
    if (vis[i] && vis[1000001 - i]) {
      symmetry++;
      continue;
    }
    if (!vis[i] && !vis[1000001 - i]) continue;
    if (!vis[i])
      S.insert(i);
    else
      S.insert(1000001 - i);
    vis[i] = true;
    vis[1000001 - i] = true;
  }
  for (long long i = 1; i <= 500000; i++) {
    if (symmetry == 0) break;
    if (vis[i] || vis[1000001 - i]) continue;
    S.insert(i);
    S.insert(1000001 - i);
    symmetry--;
  }
  cout << S.size() << '\n';
  for (auto it = S.begin(); it != S.end(); it++) cout << (*it) << " ";
}
int main(int argc, char* argv[]) {
  ControlIO(argc, argv);
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  Input();
  TimerStart();
  Solve();
  TimerStop();
  return 0;
}
void ControlIO(int argc, char* argv[]) {}
void TimerStart() {}
void TimerStop() {}
void Exit() {
  TimerStop();
  exit(0);
}
