#include <bits/stdc++.h>
using namespace std;

#define ms(s, n) memset(s, n, sizeof(s))
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define FORd(i, a, b) for (int i = (a) - 1; i >= (b); i--)
#define FORall(it, a) for (__typeof((a).begin()) it = (a).begin(); it != (a).end(); it++)
#define sz(a) int((a).size())
#define present(t, x) (t.find(x) != t.end())
#define all(a) (a).begin(), (a).end()
#define uni(a) (a).erase(unique(all(a)), (a).end())
#define pb push_back
#define pf push_front
#define mp make_pair
#define fi first
#define se second
#define prec(n) fixed<<setprecision(n)
#define bit(n, i) (((n) >> (i)) & 1)
#define bitcount(n) __builtin_popcountll(n)
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int, int> pi;
typedef vector<int> vi;
typedef vector<pi> vii;
const int MOD = (int) 1e9 + 7;
const int MOD2 = 1007681537;
const int INF = (int) 1e9;
const ll LINF = (ll) 1e18;
const ld PI = acos((ld) -1);
const ld EPS = 1e-9;
inline ll gcd(ll a, ll b) {ll r; while (b) {r = a % b; a = b; b = r;} return a;}
inline ll lcm(ll a, ll b) {return a / gcd(a, b) * b;}
inline ll fpow(ll n, ll k, int p = MOD) {ll r = 1; for (; k; k >>= 1) {if (k & 1) r = r * n % p; n = n * n % p;} return r;}
template<class T> inline int chkmin(T& a, const T& val) {return val < a ? a = val, 1 : 0;}
template<class T> inline int chkmax(T& a, const T& val) {return a < val ? a = val, 1 : 0;}
inline ll isqrt(ll k) {ll r = sqrt(k) + 1; while (r * r > k) r--; return r;}
inline ll icbrt(ll k) {ll r = cbrt(k) + 1; while (r * r * r > k) r--; return r;}
inline void addmod(int& a, int val, int p = MOD) {if ((a = (a + val)) >= p) a -= p;}
inline void submod(int& a, int val, int p = MOD) {if ((a = (a - val)) < 0) a += p;}
inline int mult(int a, int b, int p = MOD) {return (ll) a * b % p;}
inline int inv(int a, int p = MOD) {return fpow(a, p - 2, p);}
inline int sign(ld x) {return x < -EPS ? -1 : x > +EPS;}
inline int sign(ld x, ld y) {return sign(x - y);}
#define db(x) cerr << #x << " = " << (x) << " ";
#define endln cerr << "\n";

const int maxn = 2e5 + 5;
int n;
vi adj[maxn];
vi radj[maxn];
int p[maxn];
int vis[maxn];
int f[maxn];

int calc(int u) {
    int& res = f[u];
    if (~res) return res;
    map<int, int> hs;
    FOR(i, 0, sz(adj[u])) {
        int v = adj[u][i];
        hs[calc(v)] = 1;
    }
    res = 0;
    while (hs[res]) res++;
    return res;
}

vi ver;
void dfs(int u) {
    vis[u] = 1;
    FOR(i, 0, sz(adj[u])) {
        int v = adj[u][i];
        if (!vis[v]) {
            dfs(v);
        }
    }
    ver.pb(u);
}

int cnt;
void rdfs(int u) {
    vis[u] = 1, cnt++;
    FOR(i, 0, sz(radj[u])) {
        int v = radj[u][i];
        if (!vis[v]) {
            rdfs(v);
        }
    }
}

void solve() {
    cin >> n;
    FOR(i, 0, n) {
        cin >> p[i], p[i]--;
        adj[p[i]].pb(i);
        radj[i].pb(p[i]);
    }
    FOR(i, 0, n) if (!vis[i]) dfs(i);
    ms(vis, 0);
    int st;
    FORd(i, n, 0) {
        int u = ver[i];
        if (!vis[u]) {
            cnt = 0, rdfs(u);
            if (cnt > 1) {
                st = u;
                break;
            }
        }
    }
    vi path;
    path.pb(st);
    int ptr = p[st];
    while (ptr != st) {
        path.pb(ptr);
        ptr = p[ptr];
    }
    static int used[maxn];
    ms(used, 0);
    FOR(i, 0, sz(path)) {
        int u = path[i];
        used[u] = 1;
    }
    vi vals(sz(path));
    ms(f, -1);
    FOR(i, 0, sz(path)) {
        int u = path[i];
        map<int, int> hs;
        FOR(j, 0, sz(adj[u])) {
            int v = adj[u][j];
            if (!used[v]) {
                hs[calc(v)] = 1;
            }
        }
        int st = 0;
        while (hs[st]) st++;
        vals[i] = st - 1;
    }
    static int g[maxn];
    FOR(x, vals[0] + 1, vals[0] + 5) {
        g[0] = x;
        FOR(i, 1, sz(path)) {
            if (g[i - 1] <= vals[i]) {
                g[i] = vals[i] + 1;
            }
            else if (g[i - 1] == vals[i] + 1) {
                g[i] = vals[i] + 2;
            }
            else {
                g[i] = vals[i] + 1;
            }
        }
        int ok = 1;
        FOR(i, 0, sz(path)) {
            int u = i;
            int v = (i + 1) % sz(path);
            if (g[v] == vals[v] + 1 || vals[v] == g[v] - 2 && g[u] == g[v] - 1) {
            }
            else {
                ok = 0;
            }
            if (g[u] == g[v]) {
                ok = 0;
            }
        }
        if (ok) {
            cout << "POSSIBLE\n";
            return;
        }
    }
    cout << "IMPOSSIBLE\n";
}

int main() {
    int JUDGE_ONLINE = 1;
    if (fopen("in.txt", "r")) {
        JUDGE_ONLINE = 0;
        assert(freopen("in.txt", "r", stdin));
        //assert(freopen("out.txt", "w", stdout));
    }
    else {
        ios_base::sync_with_stdio(0), cin.tie(0);
    }
    solve();
    if (!JUDGE_ONLINE) {
        //cout << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << "ms\n";
    }
    return 0;
}

Measured memory_bytes: 34808