#include <bits/stdc++.h>
#define ll long long
#define INF 1000000005
#define MOD 1000000007
#define EPS 1e-10
#define rep(i,n) for(int i=0;i<(int)(n);++i)
#define rrep(i,n) for(int i=(int)(n)-1;i>=0;--i)
#define srep(i,s,t) for(int i=(int)(s);i<(int)(t);++i)
#define each(a,b) for(auto& (a): (b))
#define all(v) (v).begin(),(v).end()
#define len(v) (int)(v).size()
#define zip(v) sort(all(v)),v.erase(unique(all(v)),v.end())
#define cmx(x,y) x=max(x,y)
#define cmn(x,y) x=min(x,y)
#define fi first
#define se second
#define pb push_back
#define show(x) cout<<#x<<" = "<<(x)<<endl
#define spair(p) cout<<#p<<": "<<p.fi<<" "<<p.se<<endl
#define sar(a,n) cout<<#a<<":";rep(pachico,n)cout<<" "<<a[pachico];cout<<endl
#define svec(v) cout<<#v<<":";rep(pachico,v.size())cout<<" "<<v[pachico];cout<<endl
#define svecp(v) cout<<#v<<":";each(pachico,v)cout<<" {"<<pachico.first<<":"<<pachico.second<<"}";cout<<endl
#define sset(s) cout<<#s<<":";each(pachico,s)cout<<" "<<pachico;cout<<endl
#define smap(m) cout<<#m<<":";each(pachico,m)cout<<" {"<<pachico.first<<":"<<pachico.second<<"}";cout<<endl

using namespace std;

typedef pair<int,int> P;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<double> vd;
typedef vector<P> vp;
typedef vector<string> vs;

const int MAX_N = 100005;

class Namori{
public:
    int V;
    vector<vector<int> > G, rG, graph;
    vector<int> deg, loop;
    vector<bool> flag, inloop;
	vector<set<int> > st;
    Namori(int node_size) : V(node_size), G(V), rG(V), deg(V, 0), flag(V, false), inloop(V, false), st(V){}
    void add_edge(int u, int v){
        G[u].push_back(v), rG[v].push_back(u);
        deg[u]++, deg[v]++;
    }
    void dfs(int u){
        loop.push_back(u);
		inloop[u] = true;
        for(int v : G[u]){
            if(flag[v]) continue;
            flag[v] = true;
            dfs(v);
        }
    }
    void build(){
        graph.resize(V);
        queue<int> que;
        for(int i = 0; i < V; i++){
            if(deg[i] == 1){
                que.push(i);
                flag[i] = true;
            }
        }
        while(!que.empty()){
            int p = que.front();
            que.pop();
            for(int v : rG[p]){
                if(flag[v]) continue;
                deg[v]--;
                //サイクルから端
                graph[v].push_back(p);
                //端からサイクル
                graph[p].push_back(v);
                if(deg[v] > 1) continue;
                que.push(v);
                flag[v] = true;
            }
        }
        for(int i = 0; i < V; i++){
            if(flag[i]) continue;
            flag[i] = true;
            dfs(i);
            break;
        }
    }
	int dfs2(int u){
		set<int> hoge;
		each(v, G[u]){
			hoge.insert(dfs2(v));
		}
		int s = 0;
		auto it = hoge.begin();
		while(it != hoge.end()){
			if(*it != s) break;
			it++, s++;
		}
		return s;
	}
	bool alloc(int num){
		vi al(V);
		al[loop[0]] = num;
		auto nst = st;
		rrep(i, len(loop)){
			nst[loop[i]].insert(al[loop[(i+1)%len(loop)]]);
			auto it = nst[loop[i]].begin();
			int s = 0;
			while(it != nst[loop[i]].end()){
				if(*it != s) break;
				it++, s++;
			}
			al[loop[i]] = s;
		}
		return (al[loop[0]] == num);
	}
	bool solve(){
		rep(i,V){
			each(v, graph[i]){
				if(!inloop[v]){
					st[i].insert(dfs2(v));
				}
			}
		}
		int s = loop[0];
		int nw = 0;
		int emp = -1;
		auto it = st[s].begin();
		while(true){
			if(it == st[s].end()){
				if(emp >= 0) break;
				emp = nw++;
				continue;
			}
			if(*it != nw){
				if(emp >= 0){
					break;
				}
				emp = nw++;
				continue;
			}
			it++, nw++;
		}
		if(alloc(emp)){
			return true;
		}else{
			return alloc(nw);
		}
	}
};

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n;
    cin >> n;
	Namori g(n);
	rep(i,n){
		int a;
		cin >> a;
		g.add_edge(a-1, i);
	}
	g.build();
	if(g.solve()){
		cout << "POSSIBLE\n";
	}else{
		cout << "IMPOSSIBLE\n";
	}
    return 0;
}


Measured memory_bytes: 82176