#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vi = vector<int>;
using vl = vector<ll>;
using pi = pair<int,int>;
using pl = pair<ll,ll>;
#define all(x) x.begin(),x.end()
#define rep(i,j,n) for (long long i = j; i < (long long)(n); i++)
const long long MOD = 1000000007;
const long long MOD2 = 998244353;
const int INF = ((1<<30)-1);
const long long LINF = (1LL<<60);
const double PI = 3.141592653589793238;
template<class T> inline void chmax(T& a, T b) {if (a < b) {a = b;}}
template<class T> inline void chmin(T& a, T b) {if (a > b) {a = b;}}

class UnionFind
  {
  private:
      vector<ll> p;
      vector<ll> rank;
  public:
      UnionFind(ll n){
          p.resize(n,-1);
          rank.resize(n,1);
      }
      ll find(ll x){
          if(p[x]<0)return x;
          return p[x]=find(p[x]);
      }
      void unite(ll x,ll y){
          x = find(x);
          y = find(y);
      if(x == y)return;
          if(rank[x] > rank[y])swap(x,y);
          if(rank[x] == rank[y])rank[y]++;
          p[y] += p[x];
          p[x] = y;
      }
      ll size(ll x){
          return -p[find(x)];
      }
      bool same(ll x, ll y){
          return find(x) == find(y);
      }
  };
  

int solve(){
    int n; cin >> n;
    vi a(n);
    rep(i,0,n)cin >> a[i];
    UnionFind uf(n);
    rep(i,0,n){
        --a[i];
        uf.unite(i , a[i]);
    }
    int ans = 0;
    rep(i,0,n){
        if(uf.find(i) != i)continue;
        ans += uf.size(i)-1;
    }
    if(ans % 2 == 0)cout << 1 << endl;
    else cout << -1 << endl;

    return 0;
}

int main(){
    cout << fixed << setprecision(10);
    ios::sync_with_stdio(0), cin.tie(0);
    int t = 1; 
    //cin >> t;
    for(int i = 0; i < t; i++){
        solve();
    }
	return 0;
}