#include <cstring>
#include <cstdio>
int fa[200005], val[200005], nxt[200005], ban;
int lst[200005], to[200005], pre[200005], tot; 
bool vis[200005];
inline void add_edge(int u, int v)
{
	to[tot] = v; 
	pre[tot] = lst[u];
	lst[u] = tot++; 
}
int getfa(int u)
{
	if (u == fa[u])
		return u; 
	return fa[u] = getfa(fa[u]); 
}
void dfs(int u)
{
	if (u == ban || lst[u] == -1)
		return; 
	for (int i = lst[u]; ~i; i = pre[i])
		dfs(to[i]); 
	for (int i = lst[u]; ~i; i = pre[i])
		vis[val[to[i]]] = true; 
	for (val[u] = 0; vis[val[u]]; val[u]++); 
	for (int i = lst[u]; ~i; i = pre[i])
		vis[val[to[i]]] = false; 
}
int main()
{
	// freopen("ARC079-F.in", "r", stdin); 
	memset(lst, -1, sizeof(lst)); 
	int n, rt; 
	scanf("%d", &n); 
	for (int i = 0; i < n; i++)
		fa[i] = i; 
	for (int i = 0; i < n; i++)
	{
		scanf("%d", nxt + i); 
		nxt[i]--; 
		if (getfa(i) == getfa(nxt[i]))
			rt = i; 
		else
		{
			fa[getfa(i)] = getfa(nxt[i]); 
			add_edge(nxt[i], i); 
		}
	}
	ban = -1;
	dfs(rt); 
	if (val[rt] == val[nxt[rt]])
	{
		ban = nxt[rt]; 
		val[nxt[rt]]++; 
		dfs(rt); 
		if (val[rt] != val[nxt[rt]] - 1)
		{
			puts("IMPOSSIBLE"); 
			return 0; 
		}
	}
	puts("POSSIBLE"); 
	return 0; 
}
