#include <iostream>
#include <cstring>
#include <algorithm>
#include <vector>
#include <string>
#include <math.h>
#include <iomanip>
#include <limits>
#include <list>
#include <queue>
#include <tuple>
#include <map>
#include <stack>
#include <set>
using namespace std;
#define MOD (long long int)(1e9+7)
#define ll long long int
#define rep(i,n) for(int i=0; i<(int)(n); i++)
#define reps(i,n) for(int i=1; i<=(int)(n); i++)
#define REP(i,n) for(int i=n-1; i>=0; i--)
#define REPS(i,n) for(int i=n; i>0; i--)
#define INF (int)(1123456789)
#define LINF (long long int)(112345678901234567)
#define chmax(a, b) a = (((a)<(b)) ? (b) : (a))
#define chmin(a, b) a = (((a)>(b)) ? (b) : (a))
#define all(v) v.begin(), v.end()

const int N = (int)3e5;

ll mpow(ll a, ll b){
  if(b==0) return 1;
  else if(b%2==0){ll memo = mpow(a,b/2); return memo*memo%MOD;}
  else return mpow(a,b-1) * a % MOD;
}
ll lpow(ll a, ll b){
  if(b==0) return 1;
  else if(b%2==0){ll memo = lpow(a,b/2); return memo*memo;}
  else return lpow(a,b-1) * a;
}
ll gcd(ll a, ll b){
  if(b==0) return a;
  else return gcd(b, a%b);
}
vector<ll> kaijo_memo;
ll kaijo(ll n){
  if(kaijo_memo.size() > n) return kaijo_memo[n];
  if(kaijo_memo.size() == 0) kaijo_memo.push_back(1);
  while(kaijo_memo.size() <= n) kaijo_memo.push_back(kaijo_memo[kaijo_memo.size()-1] * kaijo_memo.size() % MOD);
  return kaijo_memo[n];
}
vector<ll> gyaku_kaijo_memo;
ll gyaku_kaijo(ll n){
  if(gyaku_kaijo_memo.size() > n) return gyaku_kaijo_memo[n];
  if(gyaku_kaijo_memo.size() == 0) gyaku_kaijo_memo.push_back(1);
  while(gyaku_kaijo_memo.size() <= n) gyaku_kaijo_memo.push_back(gyaku_kaijo_memo[gyaku_kaijo_memo.size()-1] * mpow(gyaku_kaijo_memo.size(), MOD-2) % MOD);
  return gyaku_kaijo_memo[n];
}

ll nCr(ll n, ll r){
  if(n == r) return 1;//0個の丸と-1個の棒みたいな時に時に効く？不安.
  if(n < r || r < 0) return 0;
  ll ret = 1;
  ret *= kaijo(n); ret %= MOD;
  ret *= gyaku_kaijo(r); ret %= MOD;
  ret *= gyaku_kaijo(n-r); ret %= MOD;
  return ret;
}

vector<vector<int>> G;
vector<bool> checked;
vector<int> ans;

int kiten(int now, int begin){
  if(checked[now]) return -1;
  checked[now] = true;
  rep(i, G[now].size()){
    int next = G[now][i];
    if(next == begin) return next;
    int ret = kiten(next, begin);
    if(ret != -1){
      return ret;
    }
  }
  return -1;
}

void ume(int now){
  if(checked[now]) return;
  /*if(now == 2){
    rep(i,5){
      cout<<ans[i]<<" ";
    }
    cout<<endl;
  }*/
  checked[now] = true;
  vector<int> chilnum;
  rep(i,G[now].size()){
    int next = G[now][i];
    ume(next);
    chilnum.push_back(ans[next]);
  }

  if(chilnum.size() == 0){
    ans[now] = 0;
  }else{
    sort(all(chilnum));
    /*if(now==2){
      rep(i,chilnum.size()){
        cout<<chilnum[i]<<" ";
      }
      cout<<endl;
    }*/
    int memo = -1;
    rep(i,chilnum.size()){
      if(memo < chilnum[i]){
        if(memo + 1 == chilnum[i]){
          memo++;
        }else{
          ans[now] = memo+1;
          return;
        }
      }
    }
    ans[now] = memo+1;
    return;
  }
}

int main(void){
  int n;cin>>n;
  vector<int> P;
  rep(i,n){
    G.push_back(vector<int>());
  }
  rep(i,n){
    int p;
    cin>>p;p--;
    G[p].push_back(i);
    P.push_back(p);
    checked.push_back(false);
    ans.push_back(-1);
  }
  /*rep(i,n){
    cout<<i<<": ";
    rep(j,G[i].size()){
      cout<<G[i][j]<<" ";
    }
    cout<<endl;
  }*/
  int start;
  rep(i,n){
    start = kiten(i, i);
    if(start != -1){
      break;
    }
  }

  rep(i,n){
    checked[i] = false;
    ans[i] = -1;
  }

  ume(start);
  cerr<<"start:"<<" "<<start<<endl;
  cerr<<ans[P[start]]<<endl;
  /*rep(i,n){
    cout<<i<<" "<<ans[i]<<endl;
  }*/
  if(ans[start] != ans[P[start]]){
    cout<<"POSSIBLE"<<endl;
    cerr<<"first"<<endl;
    return 0;
  }
  int p_ans = ans[P[start]];

  rep(i,n){
    checked[i] = false;
    ans[i] = -1;
  }

  ans[start] = p_ans;
  ume(start);
  ans[start] = p_ans;
  vector<int> childnum;
  rep(i, G[start].size()){
    int next = G[start][i];
    childnum.push_back(ans[next]);
  }
  sort(all(childnum));

  int memo = -1;
  rep(i,childnum.size()){
    if(memo < childnum[i]){
      if(memo + 1 == childnum[i]){
        memo++;
      }else{
        break;
      }
    }
  }
  memo++;
  cerr<<endl;
  /*rep(i,n){
    cout<<i<<" "<<ans[i]<<endl;
  }*/
  if(ans[start] == memo){
    cout<<"POSSIBLE"<<endl;
  }else{
    cout<<"IMPOSSIBLE"<<endl;
  }
  return 0;
}


Measured memory_bytes: 26220