#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using ll = long long;
using ld = long double;
using P = pair<int, int>;
using Graph = vector<vector<ll>>;
using vi = vector<int>;
using vl = vector<long>;
using vll = vector<long long>;
using vvi = vector<vi>;
using vvl = vector<vl>;
using vvll = vector<vll>;
using vs = vector<string>;
using vc = vector<char>;
using vvc = vector<vc>;
using pll = pair<long long, long long>;
using vpll = vector<pll>;
using mint = modint1000000007;
const long double EPS = 1e-18;
const long long INF = 1e18;
const long double PI = acos(-1.0L);
#define reps(i, a, n) for (ll i = (a); i < (ll)(n); i++)
#define rep(i, n) for (ll i = (0); i < (ll)(n); i++)
#define rrep(i, n) for (ll i = (1); i < (ll)(n + 1); i++)
#define repd(i, n) for (ll i = n - 1; i >= 0; i--)
#define rrepd(i, n) for (ll i = n; i >= 1; i--)
#define ALL(n) begin(n), end(n)
#define IN(a, x, b) (a <= x && x < b)
#define INIT                          \
    std::ios::sync_with_stdio(false); \
    std::cin.tie(0);
template <class T>
inline T CHMAX(T& a, const T b) {
    return a = (a < b) ? b : a;
}
template <class T>
inline T CHMIN(T& a, const T b) {
    return a = (a > b) ? b : a;
}

int main() {
    ll N;
    cin >> N;
    vll S(N);
    rep(i, N) { cin >> S[i]; }
    dsu uf(N);
    rep(i, N) { uf.merge(i, S[i] - 1); }
    auto ans = uf.groups();
    ll pp = 0;

    for (auto x : ans) {
        pp += x.size() - 1;
    }

    if (pp % 2 == 0) {
        cout << 1 << endl;
    } else {
        cout << -1 << endl;
    }
}