#include <bits/stdc++.h>

#define FOR(i,a,b) for(int i= (a); i<((int)b); ++i)
#define RFOR(i,a) for(int i=(a); i >= 0; --i)
#define FOE(i,a) for(auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define DUMP(x)  cerr << #x << " = " << (x) << endl;
#define SUM(x) std::accumulate(ALL(x), 0LL)
#define MIN(v) *std::min_element(v.begin(), v.end())
#define MAX(v) *std::max_element(v.begin(), v.end())
#define EXIST(v,x) (std::find(v.begin(), v.end(), x) != v.end())
#define BIT_ON(bit, i) (((bit >> i) & 1) > 0)

typedef long long LL;
template<typename T> std::vector<T> make_v(size_t a){return std::vector<T>(a);}
template<typename T,typename... Ts> auto make_v(size_t a, Ts... ts){ return std::vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));}    // C++14
template<typename T,typename V> typename std::enable_if<std::is_class<T>::value==0>::type fill_v(T &t,const V &v){t=v;}
template<typename T,typename V> typename std::enable_if<std::is_class<T>::value!=0>::type fill_v(T &t,const V &v){for(auto &e:t) fill_v(e,v);}
template<class T> inline T ceil(T a, T b) { return (a + b - 1) / b; }
template<class T> inline void print(T x) { std::cout << x << std::endl; }
template<class T> inline void print_vec(const std::vector<T> &v) { for (int i = 0; i < v.size(); ++i) {  if (i != 0) {std::cout << " ";} std::cout << v[i];} std::cout << "\n"; }
template<class T> inline bool inside(T y, T x, T H, T W) {return 0 <= y and y < H and 0 <= x and x < W; }
template<class T> inline double euclidean_distance(T y1, T x1, T y2, T x2) { return sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2)); }
template<class T> inline double manhattan_distance(T y1, T x1, T y2, T x2) { return abs(x1 - x2) + abs(y1 - y2); }
template<typename T> T &chmin(T &a, const T &b) { return a = std::min(a, b); }
template<typename T> T &chmax(T &a, const T &b) { return a = std::max(a, b); }

template<class T> inline std::vector<T> unique(std::vector<T> v) {
    sort(v.begin(), v.end());
    v.erase(unique(v.begin(), v.end()), v.end());
    return v;
}
// 初項s, 交差dのn個の数列の和
long long sum_of_arithmetic_progression(long long s, long long d, long long n) {
    return n * (2 * s + (n - 1) * d) / 2;
}

// 2のべき乗数かどうか
bool is_power_of_two(long long x) {
    return !(x & (x - 1));
}

// aとbの最大公約数 O(log min(a, b))
long long gcd(long long a, long long b) {
    if (b == 0) { return a; }
    return gcd(b, a % b);
}

// 数列vの最大公約数 O(N log d)
long long gcd(std::vector<long long> &v) {
    long long ans = v[0];
    for (int i = 1; i < v.size(); ++i) {
        ans = gcd(ans, v[i]);
    }
    return ans;
}

// aとbの最小公倍数 O(log min(a, b))
long long lcm(long long a, long long b) {
    long long g = gcd(a, b);
    return a / g * b;
}

const int INF = 1 << 30;
const double EPS = 1e-9;
const long double PI = acos(-1.0);
const std::vector<int> dy2 = {0, 1}, dx2 = {1, 0};  // 右，下
const std::vector<int> dy4 = {0, -1, 0, 1}, dx4 = {1, 0, -1, 0};    // 右，上，左，下
const std::vector<int> dy8 = { 0, -1, 0, 1, 1, -1, -1, 1 }, dx8 = { 1, 0, -1, 0, 1, 1, -1, -1 };


using namespace std;


void dfs(int u, vector<int> &ans, vector<vector<int>> &from, vector<vector<int>> &to) {

    FOE(v, to[u]) {
        if (ans[v] == -1) {
            unordered_set<int> s;
            bool ok = true;
            FOE(x, from[v]) {
                s.insert(ans[x]);
                if (ans[x] == -1) {
                    ok = false;
                }
            }

            if (ok) {
                FOR(i, 0, s.size() + 2) {
                    if (s.find(i) == s.end()) {
                        ans[v] = i;
                        dfs(v, ans, from, to);
                        break;
                    }
                }
            }
        }
    }
}
bool func(int u, vector<int> &ans, vector<vector<int>> &from, vector<vector<int>> &to) {
    int v = to[u][0];
    while (true) {
        unordered_set<int> s;
        FOE(x, from[v]) {
            s.insert(ans[x]);
        }

        FOR(i, 0, s.size() + 2) {
            if (s.find(i) == s.end()) {
                if (ans[v] != -1) {
                    return ans[v] == i;
                }
                ans[v] = i;
                break;
            }
        }

        v = to[v][0];
    }
}

bool check(vector<int> &ans, vector<vector<int>> &from) {
    FOR(i, 0, ans.size()) {
        unordered_set<int> s;
        FOE(x, from[i]) {
            if (ans[x] == -1) {
                return false;
            }
            s.insert(ans[x]);
        }

        FOR(j, 0, s.size() + 1) {
            if (s.find(j) == s.end()) {
                if (j != ans[i]) {
                    return false;
                }
                break;
            }
        }
    }
    return true;
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    LL N;
    cin >> N;
    auto from = make_v<int>(N, 0);
    auto to = make_v<int>(N, 0);
    FOR(i, 0, N) {
        int P;
        cin >> P;
        P--;
        from[P].emplace_back(i);
        to[i].emplace_back(P);
    }

    auto ans = make_v<int>(N);
    fill_v(ans, -1);

    FOR(i, 0, N) {
        if (from[i].empty()) {
            ans[i] = 0;
        }
    }
    FOR(i, 0, N) {
        if (ans[i] == 0) {
            dfs(i, ans, from, to);
        }
    }

    int start = -1;
    FOR(i, 0, N) {
        if (ans[i] == -1) {
            if (start == -1) {
                start = i;
            }
            if (from[i].size() != 1) {
                start = i;
            }
        }
    }
    if (start == -1) {
        if (check(ans, from)) {
            print("POSSIBLE");
        }
        else {
            print("IMPOSSIBLE");
        }
        return 0;
    }


    set<int> c;
    FOE(u, from[start]) {
        c.insert(ans[u]);
    }

    FOR(i, 0, c.size() + 2) {
        if (c.find(i) == c.end()) {
            vector<int> ans2(ans);
            ans2[start] = i;
            if (func(start, ans2, from, to)) {
                if (check(ans2, from)) {
                    print("POSSIBLE");
                    return 0;
                }
            }
        }
    }


    print("IMPOSSIBLE");



    return 0;
}


Measured memory_bytes: 22400