#include<bits/stdc++.h>
//using namespace std;
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#define rep(i,j,n) for(ll i=(ll)(j);i<(ll)(n);i++)
#define REP(i,j,n) for(ll i=(ll)(j);i<=(ll)(n);i++)
#define per(i,j,n) for(ll i=(ll)(j);(ll)(n)<=i;i--)
#define ll long long
#define ALL(a) (a).begin(),(a).end()
#define disup(A,key) distance(A.begin(),upper_bound(ALL(A),(ll)(key)))
#define dislow(A,key) distance(A.begin(),lower_bound(ALL(A),(ll)(key)))
#define pb emplace_back
#define mp std::make_pair
//#define endl "\n"
//
using std::endl;
using std::cin;
using std::cout;
using std::vector;
using std::string;
using std::upper_bound;
using std::lower_bound;
using vi=vector<ll>;
using vii=vector<vi>;
using pii=std::pair<ll,ll>;
//
constexpr ll MOD=1e9+7;
//constexpr ll MOD=998244353; 
//constexpr ll MOD=10000000;
//constexpr ll MOD=1e4;
constexpr ll MAX=3e6;
constexpr ll INF=(1ll<<60);
template<class T>
class prique :public std::priority_queue<T, std::vector<T>, std::greater<T>> {};
template<typename T>
struct Segment_tree{
    ll N;
    T mem;
    vector<T> node;
    Segment_tree(vector<T> &X,T m):mem(m){
        ll sz=X.size();
        N=1;
        while(N<sz) N*=2;
        node.resize(2*N-1,mem);
        rep(i,0,sz) node[N-1+i]=X[i];
        per(i,N-2,0){
            node[i]=Compare(node[i*2+1],node[i*2+2]);
        }
    }
    T Compare(T &A,T &B){
        return A+B;
    }
    void update(ll X,T val){
        X+=N-1;
        node[X]=val;
        while(X>0){
            X=(X-1)/2;
            node[X]=Compare(node[X*2+1],node[X*2+2]);
        }
    }
    T Query(ll a,ll b,ll now,ll l,ll r){ //[a,b),[l,r)
        if(r<0) r=N;
        if(r<=a||b<=l) return mem;
        if(a<=l&&r<=b) return node[now];
        auto vl=Query(a,b,now*2+1,l,(l+r)/2),vr=Query(a,b,now*2+2,(l+r)/2,r);
        return Compare(vl,vr);
    }
};
struct Binary_indexed_tree{
    int N;
    vi bit;
    Binary_indexed_tree(int n):N(n){
        bit.resize(N+1,0);
    }
    void add(int x,int a){
        for(x;x<=N;x+=(x&-x)) bit[x]+=a;
    }
    ll sum(int x){
        ll ret=0;
        for(x;x>0;x-=(x&-x)) ret+=bit[x];
        return ret;
    }
    ll lower_bound(ll X){
        if(sum(N)<X) return -1;
        ll ret=0,memo=1,sum=0;
        while(memo*2<=N) memo*=2;
        while(memo>0){
            if(memo+ret<=N&&sum+bit[memo+ret]<X){
                sum+=bit[memo+ret];
                ret+=memo;
            }
            memo/=2;
        }
        return ret+1;
    }
};
struct Union_Find{
    ll N;
    vi par;
    vi siz;
    Union_Find(int n):N(n){
        par.resize(N);
        siz.resize(N,1);
        rep(i,0,N) par[i]=i;
    }
    ll root(ll X){
        if(par[X]==X) return X;
        return par[X]=root(par[X]);
    }
    bool same(ll X,ll Y){
        return root(X)==root(Y);
    }
    void unite(ll X,ll Y){
        X=root(X);
        Y=root(Y);
        if(X==Y) return;
        par[X]=Y;
        siz[Y]+=siz[X];
        siz[X]=0;
    }
    ll size(ll X){
        return siz[root(X)];
    }
};
long long modpow(long long a, long long n, long long mod) {
    long long res = 1;
    while (n > 0) {
        if (n & 1) res = res * a % mod;
        a = a * a % mod;
        n >>= 1;
    }
    return res;
}
vi fac,finv,inv;
void COMinit() {
    fac.resize(MAX);
    finv.resize(MAX);
    inv.resize(MAX);
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < MAX; i++){
        fac[i] = fac[i - 1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
        finv[i] = finv[i - 1] * inv[i] % MOD;
    }
}
ll COM(ll n,ll r){
    if(n<r||n<0||r<0) return 0;
    return fac[n]*finv[r]%MOD*finv[n-r]%MOD;
}
void dfs(ll X,vii &edge,ll N,vector<bool> &seen,vector<bool> &finished,ll &x,ll &y){
    seen[X]=1;
    rep(i,0,edge[X].size()){
        ll Y=edge[X][i];
        if(!seen[Y]) dfs(Y,edge,N,seen,finished,x,y);
        else if(!finished[Y]&&x==-1){
            x=X,y=Y;
        }
    }
    finished[X]=1;
}
int main(){
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    ll N; cin>>N;
    vii edge(N);
    vii edge2(N);
    vi cnt(N),grundy(N,-1);
    rep(i,0,N){
        ll X; cin>>X;
        edge[i].pb(X-1);
        cnt[X-1]++;
        edge2[X-1].pb(i);
    }
    vector<bool> seen(N),finished(N);
    ll x=-1,y=-1;
    dfs(0,edge,N,seen,finished,x,y);
    string ans="IMPOSSIBLE";
    std::stack<ll> stack;
    cnt[y]--;
    rep(i,0,N){
        if(!cnt[i]) stack.push(i);
    }
    vi C;
    while(!stack.empty()){
        ll X=stack.top(); stack.pop();
        C.pb(X);
        rep(i,0,edge[X].size()){
            ll Y=edge[X][i];
            if(X==x&&Y==y) continue;
            cnt[Y]--;
            if(!cnt[Y]) stack.push(Y);
        }
    }
    rep(i,0,N){
        ll X=C[i];
        vi D(1,-1);
        rep(i,0,edge2[X].size()){
            ll Y=edge2[X][i];
            if(X==y&&Y==x) continue;
            D.pb(grundy[Y]);
        }
        sort(ALL(D));
        D.pb(D[D.size()-1]+2);
        rep(j,1,D.size()){
            if(D[j]-D[j-1]>1){
                grundy[X]=D[j-1]+1;
                break;
            }
        }
    }
    if(grundy[y]!=grundy[x]) ans="POSSIBLE";
    ll memo=grundy[y];
    rep(i,0,N) grundy[i]=-1;
    rep(i,0,N){
        ll X=C[i];
        vi D(1,-1);
        rep(i,0,edge2[X].size()){
            ll Y=edge2[X][i];
            if(X==y&&Y==x) continue;
            D.pb(grundy[Y]);
        }
        sort(ALL(D));
        D.pb(D[D.size()-1]+2);
        rep(j,1,D.size()){
            if(D[j]-D[j-1]>1){
                grundy[X]=D[j-1]+1;
                break;
            }
        }
        if(X==y){
            D.pop_back();
            D.pb(grundy[X]);
            sort(ALL(D));
            D.pb(D[D.size()-1]+2);
            rep(j,1,D.size()){
                if(D[j]-D[j-1]>1){
                    grundy[X]=D[j-1]+1;
                    break;
                }
            }
        }
    }
    if(memo==grundy[x]) ans="POSSIBLE";
    cout<<ans<<endl;
}


Measured memory_bytes: 47372