#include<bits/stdc++.h>

#define HEAP priority_queue
#define rep(i, n) for(int i = 0, _end_ = (n); i < _end_; ++i)
#define per(i, n) for(int i = (n) - 1; i >= 0 ; --i)
#define forn(i, l, r) for(int i = (l), _end_ = (r); i <= _end_; ++i)
#define nrof(i, r, l) for(int i = (r), _end_ = (l); i >= _end_; --i)
#define FOR(a, b) for(auto (a): (b))
#define mp make_pair
#define mt make_tuple
#define pb push_back
#define X first
#define Y second
#define eps 1e-6
#define pi 3.1415926535897932384626433832795
#define SZ(x) (int)x.size()
#define ALL(x) x.begin(), x.end()

using namespace std;

typedef long long LL;
typedef double flt;
typedef vector<int> vi;
typedef vector<LL> vl;
typedef pair<int,int> pii;
typedef pair<int,LL> pil;
typedef pair<LL,int> pli;
typedef pair<LL,LL> pll;
typedef vector<pil> vil;
typedef vector<pii> vii;
typedef vector<pli> vli;
typedef vector<pll> vll;

const int iinf = 1e9 + 7;
const int oo = 0x3f3f3f3f;
const LL linf = 1ll << 60;
const flt dinf = 1e60;

template <typename T> inline void scf(T &x)
{
	bool f = 0;
	x = 0;
	char c = getchar();
	while(c != '-' && (c < '0' || c > '9')) c = getchar();
	if(c == '-') f = 1, c = getchar();
	while(c >= '0' && c <= '9') x = x * 10 + c - '0', c = getchar();
	if(f) x = -x;
	return;
}

template <typename T1, typename T2> void scf(T1 &x, T2 &y)
{
	scf(x);
	return scf(y);
}

template <typename T1, typename T2, typename T3> void scf(T1 &x, T2 &y, T3 &z)
{
	scf(x);
	scf(y);
	return scf(z);
}

template <typename T1, typename T2, typename T3, typename T4> void scf(T1 &x, T2 &y, T3 &z, T4 &w)
{
	scf(x);
	scf(y);
	scf(z);
	return scf(w);
}

inline char mygetchar()
{
	char c = getchar();
	while(c == ' ' || c == '\n') c = getchar();
	return c;
}

template <typename T> inline bool chkmax(T &x, const T &y)
{
	return y > x ? x = y, 1 : 0;
}

template <typename T> inline bool chkmin(T &x, const T &y)
{
	return y < x ? x = y, 1 : 0;
}

#ifdef King_George
#define DEBUG
#define debug(...) fprintf(stderr, __VA_ARGS__)
#else
#define debug(...)
#endif

//---------------------------------------------------------head----------------------------------------------------

const int maxn = 2e5 + 100;

int n, rem, m;
int deg[maxn], q[maxn], sg[maxn], cyc[maxn];
bool vis[maxn], onc[maxn];
vi g[maxn], rg[maxn];

void dfs(int u)
{
	vis[u] = 1;
	for(int v: g[u]) if(!vis[v]) dfs(v); else
	{
		rem = u;
		for(int i = rg[u][0]; i != u; i = rg[i][0]) cyc[m++] = i, onc[i] = 1;
		cyc[m++] = u;
		onc[u] = 1;
	}
	return;
}

bool check()
{
	for(int v = rg[rem][0]; v != rem; v = rg[v][0])
	{
		static set<int> s;
		s.clear();
		for(int w: g[v]) s.insert(sg[w]);
		sg[v] = 0;
		while(s.find(sg[v]) != s.end()) ++sg[v];
#ifdef DEBUG
		debug("v = %d %d\n", v, sg[v]);
		puts("s:");
		for(int x: s) printf("%d ", x); puts("");
		puts("g:");
		for(int x: g[v]) printf("%d ", x); puts("");
#endif
	}
	int fuck = sg[rem];
	int v = rem;
	static set<int> s;
	s.clear();
	for(int w: g[v]) s.insert(sg[w]);
	sg[v] = 0;
	while(s.find(sg[v]) != s.end()) ++sg[v];
	return sg[v] == fuck;
}

void work(int root)
{
	dfs(root);
	int v = rem;
	static set<int> s;
	s.clear();
	for(int w: g[v]) if(~sg[w]) s.insert(sg[w]);
	sg[v] = 0;
	while(s.find(sg[v]) != s.end()) ++sg[v];
	int fuck = sg[v];
	debug("%d\t%d\n", rem, sg[rem]);
	if(check())
	{
		puts("POSSIBLE");
		return;
	}
	debug("here\n");
	rem = cyc[m - 2];
	sg[rem] = fuck;
	debug("%d\t%d\n", rem, sg[rem]);
	if(check())
	{
		puts("POSSIBLE");
		return;
	}
	puts("IMPOSSIBLE");
	return;
}

int main()
{
	scf(n);
	forn(i, 1, n)
	{
		sg[i] = -1;
		int x; scf(x);
		g[x].pb(i);
		++deg[x];
		rg[i].pb(x);
	}
	int fnt, rar;
	fnt = rar = 0;
	forn(i, 1, n) if(!deg[i]) q[rar++] = i, sg[i] = 0;
	while(fnt != rar)
	{
		int u = q[fnt++];
		for(int v: rg[u]) if(!--deg[v])
		{
			static set<int> s;
			s.clear();
			for(int w: g[v]) s.insert(sg[w]);
			sg[v] = 0;
			while(s.find(sg[v]) != s.end()) ++sg[v];
			q[rar++] = v;
		}
	}
	debug("OK\n");
	forn(u, 1, n) if(!~sg[u])
	{
		work(u);
		exit(0);
	}
	puts("POSSIBLE");
	return 0;
}

Measured memory_bytes: 32384