#include <bits/stdc++.h>
using namespace std;
vector<long long int> primes;
vector<bool> is_prime;
void sieve() {
  is_prime.resize(100000 + 1, true);
  is_prime[0] = is_prime[1] = false;
  for (long long int i = 2; i * i <= 100000; i++)
    if (is_prime[i])
      for (long long int j = i * i; j <= 100000; j += i) is_prime[j] = false;
  for (long long int i = 2; i <= 100000; i++)
    if (is_prime[i]) primes.push_back(i);
}
template <typename T>
bool isPrime(T n) {
  if (n <= 1)
    return false;
  else if (n == 2 || n == 3)
    return true;
  else if (n % 2 == 0 || n % 3 == 0)
    return false;
  T x = 5;
  T p = 2;
  while (x * x <= n) {
    if (n % x == 0) return true;
    x += p;
    p = 6 - p;
  }
  return true;
}
template <typename T>
T binpow(T x, T n, T mod = 1000000007) {
  T ans = 1;
  x = x % mod;
  while (n >= 1) {
    if (n & 1) ans = (ans * x) % mod;
    x = (x * x) % mod;
    n >>= 1;
  }
  return ans;
}
template <typename T>
T gcd(T a, T b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
class Graph {
 public:
  int n;
  vector<vector<int> > adj;
  bool directed;
  Graph(int n, bool directed = false) {
    this->n = n;
    adj.resize(n + 1);
    this->directed = directed;
  }
  void add_edge(int u, int v) {
    adj[u].push_back(v);
    if (!directed) adj[v].push_back(u);
  }
};
class DisjointSets {
 public:
  int n;
  vector<int> parent;
  vector<int> rank;
  vector<int> sz;
  DisjointSets(int n, bool Rank = true) {
    this->n = n;
    parent.resize(n + 1, 0);
    (Rank ? rank.resize(n + 1, 1) : sz.resize(n + 1, 1));
  }
  int find(int i) {
    if (i != parent[i]) parent[i] = find(parent[i]);
    return parent[i];
  }
  void UnionByRank(int i, int j) {
    i = find(i);
    j = find(j);
    if (i == j) return;
    if (rank[i] < rank[j]) swap(i, j);
    parent[j] = i;
    rank[i] += rank[j];
  }
  void UnionBySize(int i, int j) {
    i = find(i);
    j = find(j);
    if (i == j) return;
    if (sz[i] < sz[j]) swap(i, j);
    parent[j] = i;
    sz[i] += sz[j];
  }
};
template <typename T>
class SegmentTree {
 public:
  vector<T> t;
  int n, H;
  void init(int n) {
    this->n = n;
    t.resize(2 * n);
    this->H = sizeof(int) * 8 - __builtin_clz(n);
  }
  void build() {
    for (int i = n - 1; i > 0; i--) t[i] = t[i << 1] + t[(i << 1) | 1];
  }
  void update(int pos, T val) {
    for (t[pos += n] += val; pos > 1; pos >>= 1)
      t[pos >> 1] = t[pos] + t[pos ^ 1];
  }
  T query(int l, int r) {
    T res = 0;
    for (l += n, r += n; l < r; l >>= 1, r >>= 1) {
      if (l & 1) res += t[l++];
      if (r & 1) res += t[--r];
    }
    return res;
  }
};
bool isSubsequenceString(string x, string y) {
  int i = 0, j = 0;
  while (i < x.length() && j < y.length()) x[i] == y[j] ? i++, j++ : i++;
  return (j == y.length());
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int n;
  cin >> n;
  vector<int> to(n + 1);
  vector<int> visited(n + 1, -1);
  for (int i = 1; i <= n; i++) cin >> to[i];
  bool f = true;
  vector<int> cycleLength;
  for (int i = 1; i <= n; i++) {
    if (visited[i] == -1) {
      int cnt = 1;
      visited[i] = 1;
      int j = to[i];
      while (visited[j] == -1) {
        visited[j] = 1;
        j = to[j];
        cnt++;
      }
      if (j != i) {
        f = false;
        break;
      } else
        cycleLength.push_back(cnt);
    }
  }
  if (!f)
    cout << "-1\n";
  else {
    for (int n = 1;; n++) {
      bool f = true;
      for (auto x : cycleLength)
        if (2 * n % x != 0) {
          f = false;
          break;
        }
      if (f) {
        cout << n << "\n";
        return 0;
      }
    }
    cout << "-1\n";
  }
}
