#include <bits/stdc++.h>
using namespace std;
#define rep(i, n)  for(long long i=0;i<(long long)(n);i++)
#define REP(i,k,n) for(long long i=k;i<(long long)(n);i++)
#define all(a) a.begin(),a.end()
#define pb emplace_back
#define eb emplace_back
#define lb(v,k) (lower_bound(all(v),k)-v.begin())
#define ub(v,k) (upper_bound(all(v),k)-v.begin())
#define fi first
#define se second
#define pi M_PI
#define PQ(T) priority_queue<T>
#define SPQ(T) priority_queue<T,vector<T>,greater<T>>
#define dame(a) {out(a);return 0;}
#define decimal cout<<fixed<<setprecision(15);
typedef long long ll;
typedef pair<ll,ll> P;
typedef tuple<ll,ll,ll> PP;
typedef tuple<ll,ll,ll,ll> PPP;
typedef multiset<ll> S;
using vi=vector<ll>;
using vvi=vector<vi>;
using vvvi=vector<vvi>;
using vp=vector<P>;
using vvp=vector<vp>;
using vb=vector<bool>;
using vvb=vector<vb>;
const ll inf=1001001001001001001;
const int INF=1001001001;
const int mod=1000000007;
const double eps=1e-10;
template<class T> bool chmin(T&a,T b){if(a>b){a=b;return true;}return false;}
template<class T> bool chmax(T&a,T b){if(a<b){a=b;return true;}return false;}
template<class T> void out(T a){cout<<a<<'\n';}
template<class T> void outp(T a){cout<<'('<<a.fi<<','<<a.se<<')'<<'\n';}
template<class T> void outvp(T v){rep(i,v.size())cout<<'('<<v[i].fi<<','<<v[i].se<<')';cout<<'\n';}
template<class T> void outvvp(T v){rep(i,v.size())outvp(v[i]);}
template<class T> void outv(T v){rep(i,v.size()){if(i)cout<<' ';cout<<v[i];}cout<<'\n';}
template<class T> void outvv(T v){rep(i,v.size())outv(v[i]);}
template<class T> bool isin(T x,T l,T r){return (l)<=(x)&&(x)<=(r);}
template<class T> void yesno(T b){if(b)out("yes");else out("no");}
template<class T> void YesNo(T b){if(b)out("Yes");else out("No");}
template<class T> void YESNO(T b){if(b)out("YES");else out("NO");}
template<class T> void noyes(T b){if(b)out("no");else out("yes");}
template<class T> void NoYes(T b){if(b)out("No");else out("Yes");}
template<class T> void NOYES(T b){if(b)out("NO");else out("YES");}
void outs(ll a,ll b){if(a>=inf-100)out(b);else out(a);}
ll gcd(ll a,ll b){if(b==0)return a;return gcd(b,a%b);}
ll modpow(ll a,ll b){a%=mod;if(b==0)return 1;if(b&1)return a*modpow(a,b-1)%mod;ll k=modpow(a,b/2);return k*k%mod;}
vi v;
vb visited,cycle;
void dfs0(int i){
    if(cycle[i])return;
    if(visited[i])cycle[i]=true;
    visited[i]=true;
    dfs0(v[i]);
}
vvi g;
vvi memo;
ll dfs(int i){
    vi v;
    for(ll x:g[i])if(!cycle[x]){
        v.pb(dfs(x));
    }
    sort(all(v));
    memo[i]=v;
    if(v.size()==0)return 0;
    if(v[0]!=0)return 0;
    rep(i,v.size()-1)if(v[i+1]>=v[i]+2)return v[i]+1;
    return v.back()+1;
}
ll solve(vi v){
    sort(all(v));
    if(v.size()==0)return 0;
    if(v[0]!=0)return 0;
    rep(i,v.size()-1)if(v[i+1]>=v[i]+2)return v[i]+1;
    return v.back()+1;
}
vi dp;
int main(){
    ll n;cin>>n;
    v=vi(n);
    g=vvi(n);
    memo=vvi(n);
    rep(i,n)cin>>v[i];
    rep(i,n)v[i]--;
    rep(i,n)g[v[i]].pb(i);
    visited=vb(n,false);
    cycle=vb(n,false);
    dfs0(0);
    dp=vi(n,-1);
    rep(i,n)if(cycle[i]&&g[i].size()>1)dp[i]=dfs(i);
    ll s=-1;
    rep(i,n)if(dp[i]!=-1)s=i;
    if(s==-1){
        if(n&1)dame("IMPOSSIBLE");
        dame("POSSIBLE");
    }
    int w=s;
    vi d=dp;
    while(true){
        ll x=v[w];
        if(d[x]!=-1){
            vi t=memo[x];
            t.pb(d[w]);
            if(x==s){
                if(solve(t)==d[x]){
                    dame("POSSIBLE");
                }
                else break;
            }
            d[x]=solve(t);
            w=x;
        }
        else{
            if(d[w]==0)d[x]=1;
            else d[x]=0;
            w=x;
        }
    }
    vi t=memo[s];
    t.pb(dp[s]);
    dp[s]=solve(t);
    w=s;
    while(true){
        ll x=v[w];
        if(dp[x]!=-1){
            vi t=memo[x];
            t.pb(dp[w]);
            if(x==s){
                if(solve(t)==dp[x]){
                    dame("POSSIBLE");
                }
                else break;
            }
            dp[x]=solve(t);
            w=x;
        }
        else{
            if(dp[w]==0)dp[x]=1;
            else dp[x]=0;
            w=x;
        }
    }
    out("IMPOSSIBLE");
}

Measured memory_bytes: 20224