White - Black - Gray DFS

//Make CSP great again
#include <bits/stdc++.h>
#define TASK "TESTCODE"
using namespace std;
const int N = 1e5;
int a[N + 1], n, res = 0, b[N + 1];
void read()
{
    cin >> n;
    for (int i = 1; i <= n; ++ i)
    {
        cin >> a[i];
    }
}
bool DFS(int u)
{
    if (b[u] == 1)
    {
        return false;
    }
    if (b[u] == 2)
    {
        return true;
    }
    b[u] = 2;
    return DFS(a[u]);
}
void reDFS(int u)
{
    b[u] = 1;
    if (b[a[u]] == 2)
    {
        reDFS(a[u]);
    }
}
void solve()
{
    for (int i = 1; i <= n; ++ i)
    {
        if (DFS(i))
        {
            ++res;
        }
        reDFS(i);
    }
    cout << res;
}
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    //freopen(TASK".INP", "r", stdin);
    //freopen(TASK".OUT", "w", stdout);
    int t = 1;
    bool typetest = false;
    if (typetest)
    {
        cin >> t;
    }
    for (int __ = 1; __ <= t; ++ __)
    {
        read();
        solve();
    }
}