C regional

#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cassert>
#include <set>
#include <cstring>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <bitset>
#include <fstream>

using namespace std;
typedef long long ll;

#ifdef DEBUG
    const int MAXN = 10;
    const int MAXLOG = 4;
    const int MAXSQRT = 7;
#else
    const int MAXN = 3e5 + 100;
    const int MAXLOG = 20;
    const int MAXSQRT = 400;
#endif

int n;
int to[MAXN];
int cnt[MAXN];
int used[MAXN];

void init() {
    memset(to, -1, sizeof(to));
    memset(cnt, 0, sizeof(cnt));
}

void solve() {
    init();
    for (int i = 0; i < n; i++) {
        int a;
        cin >> a;
        if (a == -1) {
            continue;
        }
        a--;
        to[i] = a;
        cnt[a]++;
    }
    set<pair<int, int>> all;
    for (int i = 0; i < n; i++) {
        all.insert(make_pair(cnt[i], i));
    }
    int ans = 0;
    while (all.size() && (*all.begin()).first == 0) {
        int v = (*all.begin()).second;
        all.erase(*all.begin());
        used[v] = 1;
        ans++;
        if (to[v] != -1) {
            used[to[v]] = 1;
            if (all.count(make_pair(cnt[to[v]], to[v]))) {
                all.erase(make_pair(cnt[to[v]], to[v]));
                if (to[to[v]] != -1) {
                    if (all.count(make_pair(cnt[to[to[v]]], to[to[v]]))) {
                        all.erase(make_pair(cnt[to[to[v]]], to[to[v]]));
                        cnt[to[to[v]]]--;
                        all.insert(make_pair(cnt[to[to[v]]], to[to[v]]));
                    }
                }
            }
        }
    }
    int cur = 0;
    for (int i = 0; i < n; i++) {
        int v = i;
        while (v != -1 && !used[v]) {
            used[v] = 1;
            cur++;
            v = to[v];
        }
        ans += (cur) / 2;
        cur = 0;
    }
    cout << ans << '\n';
}

signed main() {
    #ifdef DEBUG
        freopen("C.in", "r", stdin);
        freopen("C.out", "w", stdout);
    #endif
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    while (cin >> n) {
        solve();
    }
}