#include<bits/stdc++.h>
#define overload(a,b,c,d,...) d
#define rep1(a) for(ll _=0;_<(ll)a;++_)
#define rep2(i,a) for(ll i=0;i<(ll)a;++i)
#define rep3(i,a,b) for(ll i=(ll)a;i<(ll)b;++i)
#define rep(...) overload(__VA_ARGS__,rep3,rep2,rep1)(__VA_ARGS__)
#define rrep1(i,a) for(ll i=(ll)a-1;i>=0;--i)
#define rrep2(i,a,b) for(ll i=(ll)b-1;i>=(ll)a;--i)
#define rrep(...) overload(__VA_ARGS__,rrep2,rrep1)(__VA_ARGS__)
#define eb emplace_back
#define pb push_back
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define lb(v,x) (lower_bound(all(v),x)-(v).begin())
#define ub(v,x) (upper_bound(all(v),x)-(v).begin())
using namespace std;
using ll=long long;
using vl=vector<ll>;
using ld=long double;
using i128=__int128_t;
template<typename T,typename U>inline static constexpr bool chmin(T&a,const U&b){return (a>b?a=b,true:false);}
template<typename T,typename U>inline static constexpr bool chmax(T&a,const U&b){return (a<b?a=b,true:false);}
static constexpr ll inf=numeric_limits<ll>::max()/2;
struct IOSetup{IOSetup(){ios::sync_with_stdio(false);cin.tie(nullptr);}};
static constexpr ll mod=998244353;
ll fac(ll n){return (n==0?1:fac(n-1)*n);}
int main(){
    ll n;cin>>n;
    vl p(n);for(auto&&e:p)cin>>e;
    vl nxt(n,-1);
    vl bef(n,-1);
    rep(i,n-1)nxt[p[i]-1]=p[i+1]-1;
    rep(i,n-1)bef[p[i+1]-1]=p[i]-1;
    set<ll>st;
    rep(i,n)st.insert(i);
    vl q;
    rep(i,n/2){
        ll a=*st.begin();
        if(nxt[a]==-1)a=*next(st.begin());
        ll b=nxt[a];
        q.eb(a);
        q.eb(b);
        if(bef[a]!=-1)nxt[bef[a]]=nxt[b];
        if(nxt[b]!=-1)bef[nxt[b]]=bef[a];
        st.erase(a);
        st.erase(b);
    }
    rep(i,n)cout<<q[i]+1<<" \n"[i==n-1];
}