#include <bits/stdc++.h>
#define REP(i, n)     for(int i = 0; i < (int)(n); ++i)
#define RREP(i, n)    for(int i = (int)(n); i-- > 0;)
#define FOR(i, l, r)  for(int i = (int)(l); i < (int)(r); ++i)
#define RFOR(i, l, r) for(int i = (int)(r); i-- > (int)(l);)
#define ALL(v)        std::begin(v), std::end(v)
using llong = long long;
using vi    = std::vector<int>;
using vvi   = std::vector<vi>;
using pii   = std::pair<int, int>;
using namespace std;
constexpr int       INF  = 1e9;
constexpr long long LINF = 1e18;
constexpr double    EPS  = 1e-10;
constexpr int       MOD  = 998'244'353;
constexpr int       MOD2 = 1e9 + 7;

template <typename C, typename Tr, typename R, typename T = std::ranges::range_value_t<R>>
    requires std::ranges::output_range<R, T>
inline auto &operator>>(std::basic_istream<C, Tr> &is, R &r) {
    for(auto &elem : r) is >> elem;
    return is;
}

template <typename C, typename Tr, std::ranges::input_range R>
    requires(!std::convertible_to<R, const char *>)
inline auto &operator<<(std::basic_ostream<C, Tr> &os, const R &r) {
    if(std::ranges::empty(r)) return os;
    auto iter = std::ranges::cbegin(r);
    const auto end = std::ranges::cend(r);
    os << *iter++;
    while(iter != end) os << " " << *iter++;
    return os;
}

#ifdef DEBUG

#include <debug>

#else

#define debug(...) static_cast<void>(0)

#endif

int main(){
    int n;
    cin>>n;

    vi p(n);
    vi deg(n,0);
    for(auto &e:p){
        cin>>e;
        if(e!=0) deg[e-1]++;
    }
    debug(deg);

    vi depth(n,-1);
    queue<int> que;
    REP(i,n){
        if(deg[i]>0) continue;
        depth[i]=0;
        que.push(i);
    }
    while(!que.empty()){
        auto u=que.front();
        que.pop();

        if(p[u]==0) continue;
        auto v=p[u]-1;
        if(depth[v]!=-1) continue;
        depth[v]=depth[u]+1;
        que.push(v);
    }
    
    int ans=0;
    for(auto e:depth){
        if(e==2) ans++;
    }

    cout<<ans<<endl;
}