#include <bits/stdc++.h>
using namespace std;
using ll = long long;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    cin >> n;
    vector<int> p(n), pr(n, -1);
    for(int i = 0; i < n; i++){
        cin >> p[i];
        p[i]--;
        if(p[i] == -1) continue;
        pr[p[i]] = i;
    }
    int ans = 0, tot = 0;
    for(int i = 0; i < n; i++){
        if(p[i] != -1) continue;
        if(pr[i] == -1) continue;
        ans += (pr[pr[i]] != -1);
        tot += pr[pr[i]] == i;
    }
    ans += tot / 2;
    cout << ans << '\n';
}