// Copyright (C) 2017 __debug.

// This program is free software; you can redistribute it and/or
// modify it under the terms of the GNU General Public License as
// published by the Free Software Foundation; version 3

// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this program; If not, see <http://www.gnu.org/licenses/>.


#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/priority_queue.hpp>

#define x first
#define y second
#define MP std::make_pair
#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(), (x).end()
#define DEBUG(...) fprintf(stderr, __VA_ARGS__)
#ifdef __linux__
#define getchar getchar_unlocked
#define putchar putchar_unlocked
#endif

using std::pair;
using std::vector;
using std::string;

typedef long long LL;
typedef pair<int, int> Pii;

const int oo = 0x3f3f3f3f;

template<typename T> inline bool chkmax(T &a, T b) { return a < b ? a = b, true : false; }
template<typename T> inline bool chkmin(T &a, T b) { return b < a ? a = b, true : false; }
string procStatus()
{
    std::ifstream t("/proc/self/status");
    return string(std::istreambuf_iterator<char>(t), std::istreambuf_iterator<char>());
}
template<typename T> T read(T &x)
{
    int f = 1;
    char ch = getchar();
    for (; !isdigit(ch); ch = getchar())
        f = (ch == '-' ? -1 : 1);
    for (x = 0; isdigit(ch); ch = getchar())
        x = 10 * x + ch - '0';
    return x *= f;
}
template<typename T> void write(T x)
{
    if (x == 0) {
        putchar('0');
        return;
    }
    if (x < 0) {
        putchar('-');
        x = -x;
    }
    static char s[20];
    int top = 0;
    for (; x; x /= 10)
        s[++top] = x % 10 + '0';
    while (top)
        putchar(s[top--]);
}
// EOT

const int MAXN = 2e5 + 5;

int N;
int P[MAXN];
vector<int> G[MAXN];

void input()
{
    read(N);
    for (int i = 1; i <= N; ++i) {
        read(P[i]);
        G[P[i]].push_back(i);
    }
}

int vis[MAXN], visclock;
bool oncycle[MAXN];
int min[MAXN], sec[MAXN];

int dfs(int u)
{
    vector<int> list;
    for (int i = 0; i < SZ(G[u]); ++i) {
        int v = G[u][i];
        if (!oncycle[v])
            list.push_back(dfs(v));
    }
    std::sort(ALL(list));
    list.push_back(+oo);
    int i = 0, p = 0;
    for (; list[p] == i; ++i, ++p) { }
    min[u] = i++;
    for (; list[p] == i; ++i, ++p) { }
    sec[u] = i;
    return min[u];
}

void solve()
{
    for (int i = 1; i <= N; ++i) {
        if (vis[i])
            continue;
        ++visclock;
        int j = i;
        for (; !vis[j]; j = P[j]) {
            vis[j] = visclock;
        }
        if (vis[j] == visclock) {
            for (; !oncycle[j]; j = P[j]) {
                oncycle[j] = true;
            }
        }
    }
    for (int i = 1; i <= N; ++i) {
        if (oncycle[i])
            dfs(i);
    }

    // for (int i = 1; i <= N; ++i) {
    //     if (oncycle[i])
    //         printf("%d: min=%d, sec=%d\n", i, min[i], sec[i]);
    // }

    std::fill(vis + 1, vis + N + 1, 0);
    for (int i = 1; i <= N; ++i) {
        if (vis[i] || !oncycle[i])
            continue;
        int last = min[i];
        vis[i] = true;
        for (int j = P[i]; j != i; j = P[j]) {
            last = (last == min[j] ? sec[j] : min[j]);
            vis[j] = true;
        }
        if (last != min[i])
            continue;
        last = sec[i];
        for (int j = P[i]; j != i; j = P[j]) {
            last = (last == min[j] ? sec[j] : min[j]);
        }
        if (last != min[i]) {
            puts("IMPOSSIBLE");
            return;
        }
    }
    puts("POSSIBLE");
}

int main()
{
#ifdef __DEBUG
    freopen("F.in", "r", stdin);
    freopen("F.out", "w", stdout);
#endif

    input();
    solve();
}

// 南国有佳人，容华若桃李。
//     -- 曹植《杂诗·南国有佳人》


Measured memory_bytes: 13440