#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) REP(i,0,n)
#define REP(i,s,e) for(int i=(s); i<(int)(e); i++)
#define repr(i, n) REPR(i, n, 0)
#define REPR(i, s, e) for(int i=(int)(s-1); i>=(int)(e); i--)
#define all(r) r.begin(),r.end()
#define rall(r) r.rbegin(),r.rend()

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;

const ll INF = 1e9;
const ll MOD = 1e9 + 7;

template<typename T> T chmax(T& a, const T& b){return a = (a > b ? a : b);}
template<typename T> T chmin(T& a, const T& b){return a = (a < b ? a : b);}


struct MaxFlow {
    struct Edge {
        int to, cap, rev;
        Edge() {};
        Edge(int to, int cap, int rev) : to(to), cap(cap), rev(rev) {};
    };
    int V;
    vector<vector<Edge>> G;
    vector<int> used;

    MaxFlow(int V) : V(V), G(V, vector<Edge>()), used(V){}

    void add_edge(int from, int to, int cap) {
        G[from].push_back({to, cap, (int)G[to].size()});
        G[to].push_back({from, 0, (int)G[from].size() - 1});
    }

    int dfs(int v, int t, int f) {
        if (v == t) return f;
        used[v] = true;
        for (int i = 0; i < (int)G[v].size(); i++) {
            Edge& e = G[v][i];
            if (!used[e.to] && e.cap > 0) {
                int d = dfs(e.to, t, min(f, e.cap));
                if (d > 0) {
                    e.cap -= d;
                    G[e.to][e.rev].cap += d;
                    return d;
                }
            }
        }
        return 0;
    }

    int max_flow(int s, int t) {
        int flow = 0;
        while (1) {
            // memset(used, 0, sizeof(used));
            used = vector<int>(V, 0);
            int f = dfs(s, t, INF);
            if (f == 0) return flow;
            flow += f;
        }
    }
};

int main(){
    int n;
    cin >> n;
    vi a(n);
    rep(i, n) cin >> a[i];
    MaxFlow f(n*2+2);
    const int sink = n*2+1, source = n*2;
    rep(i, n) f.add_edge(source, i, 1);
    rep(i, n) f.add_edge(n+i, sink, 1);
    rep(i, n) rep(j, n) if(j != a[i]) {
        f.add_edge(i, n+j, 1);
    }
    int ans = f.max_flow(source, sink);
    if(ans != n) {
        cout << -1 << '\n';
        return 0;
    }
    rep(i, n) for(auto&&e : f.G[i]) {
        if(e.cap == 0) {
            cout << e.to-n << '\n';
            break;
        }
    }
    return 0;
}