10B

#include <iostream>
#include <vector>
#include <unordered_map>

using namespace std;

const int INF = 1e9 + 1;
const int MAXN = 100000;

vector<int> g[MAXN];
int pr[MAXN];
bool dp[MAXN];

void dfs(int ver) {
    for (const auto& i : g[ver]) {
        if (i != pr[ver]) {
            dfs(i);
            if (!dp[i]) {
                dp[ver] = true;
            }
        }
    }
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    int n;
    cin >> n;
    int root = 0;
    for (int i = 0; i < n; ++i) {
        cin >> pr[i];
        --pr[i];
        if (pr[i] == -1) {
            root = i;
        } else {
            g[pr[i]].emplace_back(i);
        }
    }
    dfs(root);
    cout << (dp[root] ? "YES" : "NO");
}