#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
 
#define ALL(obj) (obj).begin(),(obj).end()
#define SPEED cin.tie(0);ios::sync_with_stdio(false);
 
template<class T> using PQ = priority_queue<T>;
template<class T> using PQR = priority_queue<T,vector<T>,greater<T>>;
 
constexpr long long MOD = (long long)1e9 + 7;
constexpr long long MOD2 = 998244353;
constexpr long long HIGHINF = (long long)1e18;
constexpr long long LOWINF = (long long)1e15;
constexpr long double PI = 3.1415926535897932384626433;
 
template <class T> vector<T> multivector(size_t N,T init){return vector<T>(N,init);}
template <class... T> auto multivector(size_t N,T... t){return vector<decltype(multivector(t...))>(N,multivector(t...));}
template <class T> void corner(bool flg, T hoge) {if (flg) {cout << hoge << endl; exit(0);}}
template <class T, class U>ostream &operator<<(ostream &o, const map<T, U>&obj) {o << "{"; for (auto &x : obj) o << " {" << x.first << " : " << x.second << "}" << ","; o << " }"; return o;}
template <class T>ostream &operator<<(ostream &o, const set<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const multiset<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const vector<T>&obj) {o << "{"; for (int i = 0; i < (int)obj.size(); ++i)o << (i > 0 ? ", " : "") << obj[i]; o << "}"; return o;}
template <class T, class U>ostream &operator<<(ostream &o, const pair<T, U>&obj) {o << "{" << obj.first << ", " << obj.second << "}"; return o;}
void print(void) {cout << endl;}
template <class Head> void print(Head&& head) {cout << head;print();}
template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {cout << head << " ";print(forward<Tail>(tail)...);}
template <class T> void chmax(T& a, const T b){a=max(a,b);}
template <class T> void chmin(T& a, const T b){a=min(a,b);}
void YN(bool flg) {cout << (flg ? "YES" : "NO") << endl;}
void Yn(bool flg) {cout << (flg ? "Yes" : "No") << endl;}
void yn(bool flg) {cout << (flg ? "yes" : "no") << endl;}

class StronglyConnectedComponents{
    int nodeNum;
    vector<vector<int>> edge,revEdge;

    vector<int> label,visited,order;
    
    void dfs(int curr){
        visited[curr] = 1;
        for(int next:edge[curr]) if(!visited[next]) dfs(next);
        order.push_back(curr);
    }

    void revDfs(int curr,int labelId){
        visited[curr] = 1;
        label[curr] = labelId;
        for(int next:revEdge[curr]) if(!visited[next]) revDfs(next,labelId);
    }

public:

	StronglyConnectedComponents(const int& nodeNum) : 
    nodeNum(nodeNum), edge(nodeNum), revEdge(nodeNum), label(nodeNum), visited(nodeNum) {
        // do nothing        
    }

	int operator[](int idx) {
		return label[idx];
	}

    void makeEdge(const int from,const int to) {
        edge[from].push_back(to);
        revEdge[to].push_back(from);
    }

    void solve(void) {
        for(int i = 0; i < nodeNum; ++i) visited[i] = 0;
        for(int i = 0; i < nodeNum; ++i) if(!visited[i]) dfs(i);
        for(int i = 0; i < nodeNum; ++i) visited[i] = 0;
        reverse(order.begin(),order.end());
        int labelId = 0;
        for(int i:order) if(!visited[i]) revDfs(i,labelId++);
    }

    void print(void) {
        for(auto labelId:label) cout << labelId << " ";
        cout << endl;
    }

};

int main() {
	int N; cin >> N;
	StronglyConnectedComponents scc(N);
	vector<vector<int>> edge(N);
	vector<vector<int>> redge(N);
	for(int i = 0; i < N; ++i) {
		int p; cin >> p; p--;
		edge[p].push_back(i);
		redge[i].push_back(p);
		scc.makeEdge(p,i);
	}
	scc.solve();
	vector<int> cnt(N,0),cycle;
	for(int i = 0; i < N; ++i) cnt[scc[i]]++;
	int root = max_element(ALL(cnt)) - cnt.begin();
	int M = cnt[root],s;
	for(int i = 0; i < N; ++i) if(scc[i]==root) s = i;
	cycle.push_back(s);
	int from = s;
	while(1){
        int to;
		for(int tmp: redge[from]){
			if(scc[tmp] != root) continue;
            to = tmp;
		}
        if(to==s) break;
        cycle.push_back(from = to);
	}
    queue<int> q;
    vector<int> depth(N,0);
    for(int i:cycle){
        q.push(i);
        while(q.size()){
            int from = q.front();
            q.pop();
            for(int to:edge[from]){
                if(scc[to]==root) continue;
                depth[to] = depth[from] + 1;
                q.push(to);
            }
        }
    }
    vector<int> ord(N);
    iota(ALL(ord),0);
    sort(ALL(ord),[&](int l,int r){return depth[l] > depth[r];});
    vector<int> grundy(N,-1);
    for(int i:ord){
        if(scc[i]==root) continue;
        set<int> st;
        for(int j:edge[i]) st.insert(grundy[j]);
        for(int j = 0; j < N; ++j) {
            if(st.count(j)) continue;
            grundy[i] = j;
            break;
        }
    }
    vector<int> cand;
    set<int> st2;
    for(int i:edge[cycle.front()]) st2.insert(grundy[i]);
    for(int i = 0; i < N && cand.size() < 3; ++i) {
        if(st2.count(i)) continue;
        cand.push_back(i);
    }
    int ans = 0;
    for(int c:cand){
        int flg = 1;
        for(int i:cycle) grundy[i] = -1;
        grundy[cycle.front()] = c;
        for(int i:cycle){
            if(i==cycle.front()) continue;
            set<int> st;
            for(int j:edge[i]) st.insert(grundy[j]);
            for(int j = 0; j < N; ++j) {
                if(st.count(j)) continue;
                grundy[i] = j;
                break;
            }
        }
        if(c == grundy[cycle.back()]) flg = 0;
        auto st3 = st2;
        st3.insert(grundy[cycle.back()]);
        for(int i = 0; i < c; ++i) if(!st3.count(i)) flg = 0;
        if(flg) ans = 1;
    }
    cout << (ans?"POSSIBLE":"IMPOSSIBLE") << endl;
    return 0;
}

Measured memory_bytes: 51572