/*

This Submission is to determine how many 120/240 min const. delivery point there are.

//info
120 req. steps <= 5
*/
#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream>
#include <random>
#include<map>
#include <iomanip>
#include <time.h>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include <cassert>
#include<fstream>
#include <unordered_map>
#include <cstdlib>
#include <complex>
#include <cctype>
#include <bitset>
using namespace std;
typedef string::const_iterator State;
#define Ma_PI 3.141592653589793
#define eps 1e-5
#define LONG_INF 1e18
#define GOLD 1.61803398874989484820458
#define MAX_MOD 1000000007LL
#define MOD 998244353LL
#define seg_size 262144
#define REP(a,b) for(long long a = 0;a < b;++a)

unsigned long xor128() {
	static unsigned long x = time(NULL), y = 362436069, z = 521288629, w = 88675123;
	unsigned long t = (x ^ (x << 11));
	x = y; y = z; z = w;
	return (w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)));
}
double dot(complex<double> a, complex<double> b) {
	return a.real() * b.real() + a.imag() * b.imag();
}
double gyaku_dot(complex<double> a, complex<double> b) {
	return a.real() * b.imag() - a.imag() * b.real();
}
double leng(complex<double> a) {
	return sqrt(a.real() * a.real() + a.imag() * a.imag());
}
double angles(complex<double> a, complex<double> b) {
	double cosine = dot(a, b) / (leng(a) * leng(b));
	double sine = gyaku_dot(a, b) / (leng(a) * leng(b));
	double kaku = acos(min((double)1.0, max((double)-1.0, cosine)));
	if (sine <= 0) {
		kaku = 2 * Ma_PI - kaku;
	}
	return kaku;
}
vector<int> convex_hull(vector<complex<double>> a) {
	vector<int> ans;
	double now_minnest = a[0].real();
	int now_itr = 0;
	REP(i, a.size()) {
		if (now_minnest > a[i].real()) {
			now_minnest = a[i].real();
			now_itr = i;
		}
	}
	ans.push_back(now_itr);
	complex<double> ba(0, 1);
	while (true) {
		int now_go = 0;
		double now_min = 0;
		double now_length = 0;
		int starter = ans[ans.size() - 1];
		for (int i = 0; i < a.size(); ++i) {
			if (i != starter) {
				double goa = angles(ba, a[i] - a[starter]);
				if (goa - now_min >= eps || (abs(goa - now_min) <= eps && (abs(a[i] - a[starter]) - now_length) >= eps)) {
					now_min = goa;
					now_go = i;
					now_length = abs(a[i] - a[starter]);
				}
			}
		}
		if (now_go == ans[0]) break;
		ans.push_back(now_go);
		ba = complex<double>(a[now_go] - a[starter]);
	}
	return ans;
}

vector<int> vertexs[300000];
int gyaku_cnt[300000];
int gyaku[300000];
int ans[300000];
int memo[300000];
vector<pair<int, int>> edges;
int n;
bool verify() {
	for (int q = 0; q < edges.size(); ++q) {
		if (ans[edges[q].first] == ans[edges[q].second])return false;
	}
	for (int i = 0; i < n; ++i) {
		set<int> hoge;
		REP(q, vertexs[i].size()) {
			hoge.insert(ans[vertexs[i][q]]);
		}
		for (int q = 0; q < ans[i]; ++q) {
			if (hoge.find(q) == hoge.end()) {
				return false;
			}
		}
	}
	return true;
}
int main() {
	iostream::sync_with_stdio(false);
	cin.tie(0);
	cin >> n;
	REP(i, n) {
		int a;
		cin >> a;
		a--;
		edges.push_back(make_pair(i, a));
		vertexs[a].push_back(i);
		gyaku_cnt[a]++;
		gyaku[i] = a;
	}
	REP(i, n) {
		ans[i] = -1;
	}
	queue<int> next;
	REP(i, n) {
		if (gyaku_cnt[i] == 0) {
			next.push(i);
		}
	}
	while (next.empty() == false) {
		int now = next.front();
		next.pop();
		set<int> neko;
		REP(q, vertexs[now].size()) {
			neko.insert(ans[vertexs[now][q]]);
		}
		for (int i = 0;; ++i) {
			if (neko.find(i) == neko.end()) {
				ans[now] = i;
				break;
			}
		}
		gyaku_cnt[gyaku[now]]--;
		if (gyaku_cnt[gyaku[now]] == 0) {
			next.push(gyaku[now]);
		}
	}
	for (int i = 0; i < n; ++i) {
		if (ans[i] != -1) continue;
		set<int> geko;
		for (int q = 0; q < vertexs[i].size(); ++q) {
			geko.insert(ans[vertexs[i][q]]);
		}
		int cnt = 0;
		for (int q = 0;cnt < 2; ++q) {
			if (geko.find(q) == geko.end()) {
				REP(j, n) {
					memo[j] = ans[j];
				}
				ans[i] = q;
				int now = gyaku[i];
				while (ans[now] == -1) {
					set<int> neko;
					REP(q, vertexs[now].size()) {
						neko.insert(ans[vertexs[now][q]]);
					}
					for (int i = 0;; ++i) {
						if (neko.find(i) == neko.end()) {
							ans[now] = i;
							break;
						}
					}
					now = gyaku[now];
				}
				if (verify() == true) {
					cout << "POSSIBLE" << endl;
					return 0;
				}
				else {
					REP(j, n) {
						ans[j] = memo[j];
					}
				}
				cnt++;
			}
		}
		break;
	}
	cout << "IMPOSSIBLE" << endl;
	return 0;
}

Measured memory_bytes: 18416