#include <bits/stdc++.h> using namespace std; const int N = 2e5 + 3; int n, ans[N], b[N], vis[N]; int main(){ cin.tie(0)->sync_with_stdio(0); cin >> n; for (int i = 1; i <= n; i++) cin >> b[i]; for (int i = n, pos = 2*n - 1, pre = 0; i >= 1; i--){ if (b[i] != pre){ ans[pos] = b[i]; vis[b[i]] = 1; pre = b[i]; } pos -= 2; } int sm = 1, bg = 0, s = 1, e = 2 * n - 1; for (int i = 2; i <= 2*n - 1; i++){ if (ans[i]) { sm++; } else{ if (sm < bg) { while (vis[s]) s++; vis[s] = 1; ans[i] = s; sm++; } else { while (vis[e]) e--; vis[e] = 1; ans[i] = e; bg++; } } } for (int i = 1; i <= 2*n - 1; i++) cout << ans[i] << " "; cout << "\n"; }