#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
const ll MOD = 1000000007LL;
int par[200000];
vector<int> out[200000];
int pos[200000];
set<int> s[200000];
int dfs(int v) {
	set<int> s;
	for (int to : out[v]) s.insert(dfs(to));
	int x = 0;
	while (s.find(x) != s.end()) x++;
	return x;
}
int main() {
	int N;
	cin >> N;
	for (int i = 0; i < N; i++) {
		cin >> par[i];
		par[i]--;
		out[par[i]].push_back(i);
	}
	vector<int> vec;
	memset(pos, -1, sizeof(pos));
	pos[0] = 0;
	vec.push_back(0);
	int now = par[0];
	while (pos[now] == -1) {
		pos[now] = vec.size();
		vec.push_back(now);
		now = par[now];
	}
	int p = pos[now];
	for (int i = p; i < vec.size(); i++) {
		int prev;
		if (i == p) prev = vec.size() - 1;
		else prev = i - 1;
		for (int to : out[vec[i]]) {
			if (to == vec[prev]) continue;
			s[vec[i]].insert(dfs(to));
		}
	}
	vec.push_back(vec[p]);
	for (int i = 0; i <= out[vec[p]].size(); i++) {
		int x = i;
		for (int j = p + 1; j < vec.size(); j++) {
			int y = 0;
			while (y == x || s[vec[j]].find(y) != s[vec[j]].end()) y++;
			x = y;
		}
		if (x == i) {
			cout << "POSSIBLE" << endl;
			return 0;
		}
	}
	cout << "IMPOSSIBLE" << endl;
}
