#include<stdio.h>
#include<vector>
#include<algorithm>
using namespace std;
int to[200000];
vector<int>frm[200000];
bool flag[220000];
bool iscirc[200000];
typedef pair<int, int>pii;
pii dfs(int node)
{
	vector<int>v;
	for (int i = 0; i < frm[node].size(); i++)
	{
		if (!iscirc[frm[node][i]])
		{
			v.push_back(dfs(frm[node][i]).first);
		}
	}
	for (int i = 0; i < v.size(); i++)flag[v[i]] = true;
	pii z;
	int c = 0;
	for (int i = 0;; i++)
	{
		if (!flag[i] && c == 0)c++, z.first = i;
		else if (!flag[i] && c == 1)
		{
			z.second = i;
			break;
		}
	}
	for (int i = 0; i < v.size(); i++)flag[v[i]] = false;
	return z;
}
void yes()
{
	printf("POSSIBLE\n");
	exit(0);
}
void no()
{
	printf("IMPOSSIBLE\n");
	exit(0);
}
int main()
{
	int num;
	scanf("%d", &num);
	for (int i = 0; i < num; i++)
	{
		scanf("%d", &to[i]);
		to[i]--;
		frm[to[i]].push_back(i);
	}
	int now = 0;
	vector<int>circ;
	for (;;)
	{
		if (flag[now])
		{
			int z = now;
			for (;;)
			{
				circ.push_back(z);
				//printf("%d\n", z + 1);
				iscirc[z] = true;
				z = to[z];
				if (z == now)break;
			}
			break;
		}
		flag[now] = true;
		now = to[now];
	}
	fill(flag, flag + num, false);
	vector<pii>v;
	for (int i = 0; i < circ.size(); i++)v.push_back(dfs(circ[i]));
	int t = v[v.size() - 1].first;
	for (int i = 0; i < v.size(); i++)
	{
		//printf("%d %d\n", v[i].first, v[i].second);
		if (v[i].first == t)t = v[i].second;
		else t = v[i].first;
	}
	if (t == v[v.size() - 1].first)yes();
	t = v[v.size() - 1].second;
	for (int i = 0; i < v.size(); i++)
	{
		if (v[i].first == t)t = v[i].second;
		else t = v[i].first;
	}
	if (t == v[v.size() - 1].second)yes();
	no();
}