#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
#define rep2(i, m, n) for (int i = (m); i < (n); ++i)
#define rep(i, n) rep2(i, 0, n)
#define drep2(i, m, n) for (int i = (m)-1; i >= (n); --i)
#define drep(i, n) drep2(i, n, 0)
#define all(a) (a).begin(), (a).end()
template<typename T = int> using V = vector<T>;
template<typename T = int> using P = pair<T, T>;
using Vi = V<int>;
using Vl = V<ll>;
using Vd = V<ld>;
using Vb = V<bool>;
using VVi = V<Vi>;
using VVl = V<Vl>;
using VVb = V<Vb>;
using Pi = P<int>;
using Pl = P<ll>;
using Pd = P<ld>;
template<typename T> vector<T> make_vec(size_t n, T a) { return vector<T>(n, a); }
template<typename... Ts> auto make_vec(size_t n, Ts... ts) { return vector<decltype(make_vec(ts...))>(n, make_vec(ts...)); }
template<typename T> inline int sz(T &x) { return x.size(); }
template<typename T> inline bool chmin(T &a, T b) { if (a > b) { a = b; return true; } return false; }
template<typename T> inline bool chmax(T &a, T b) { if (a < b) { a = b; return true; } return false; }
template<typename T1, typename T2> istream &operator>>(istream &is, pair<T1, T2> &p) { is >> p.first >> p.second; return is; }
template<typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &p) { os << '(' << p.first << ", " << p.second << ')'; return os; }
template<typename T> istream &operator>>(istream &is, vector<T> &v) { for (auto &e : v) is >> e; return is; }
template<typename T> ostream &operator<<(ostream &os, const vector<T> &v) { for (auto &e : v) os << e << ' '; return os; }
template<typename T> inline int count_between(vector<T> &a, T l, T r) { return lower_bound(all(a), r) - lower_bound(all(a), l); } // [l, r)
inline int cDiv(const ll x, const ll y) { return (x+y-1) / y; } // ceil(x/y)
inline int fLog2(const ll x) { assert(x > 0); return 63-__builtin_clzll(x); } // floor(log2(x))
inline int cLog2(const ll x) { assert(x > 0); return (x == 1) ? 0 : 64-__builtin_clzll(x-1); } // ceil(log2(x))
inline int popcount(const ll x) { return __builtin_popcountll(x); }
inline void fail() { cout << -1 << '\n'; exit(0); }
struct fast_ios { fast_ios(){ cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
// const int INF  = 1<<30;
// const ll INFll = 1ll<<60;
// const ld EPS   = 1e-10;
// const ld PI    = acos(-1.0);
// const int MOD  = int(1e9)+7;
// const int MOD  = 998244353;


VVi G;
Vi grn;

int dfs(int u) {
  if (grn[u] != -1) return grn[u];

  int d = sz(G[u]);
  Vb seen(d+1);
  for (int v : G[u]) {
    int g = dfs(v);
    if (g <= d) seen[g] = true;
  }
  rep(i, d+1) if (!seen[i]) return grn[u] = i;
  return -1;
}



int main() {
  int n; cin >> n;
  Vi p(n); cin >> p;
  rep(i, n) --p[i];

  int x = 0;
  Vb seen(n);
  while (!seen[x]) {
    seen[x] = true;
    x = p[x];
  }

  Vb cyc(n);
  while (!cyc[x]) {
    cyc[x] = true;
    x = p[x];
  }

  G.assign(n, Vi());
  grn.assign(n, -1);
  rep(i, n) G[p[i]].push_back(i);

  Vi rs;
  rep(i, n) if (cyc[i]) {
    for (auto j : G[i]) if (!cyc[j]) rs.push_back(j);
  }

  for (auto r : rs) dfs(r);

  int s = -1;
  int cm = n;
  rep(i, n) if (cyc[i]) {
    int d = sz(G[i]);
    if (d < cm) s = i, cm = d;
  }

  set<int> S;
  rep(i, n) S.insert(i);
  for (auto v : G[s]) S.erase(grn[v]);

  if (sz(S) > 2) S.erase(next(S.begin(), 2), S.end());


  for (auto g : S) {
    grn[s] = g;
    int u = p[s];
    while (u != s) {
      dfs(u);
      u = p[u];
    }
    grn[s] = -1;
    if (g == dfs(s)) {
      cout << "POSSIBLE\n";
      return 0;
    }
    u = p[u];
    while (u != s) {
      grn[u] = -1;
      u = p[u];
    }
  }
  
  cout << "IMPOSSIBLE\n";
}


Measured memory_bytes: 23924