#include <bits/stdc++.h>
using namespace std;
int dfs(int now, int par, vector<vector<int>>&G, vector<int>&d){
    if(d[now] != -1) return d[now];
    d[now] = 0;
    for(int t: G[now]){
        if(t == now) continue;
        if(t == par) continue;
        d[now] = max(d[now], dfs(t, now, G, d));
    }
    d[now] += 1;
    return d[now];
}
int main(){
    int N;
    cin >> N;
    vector<int> H(N), W(N);
    vector<vector<int>>  G(200010);
    set<int> s;
    vector<int> cnt(200010, -1);
    for(int i = 0; i < N; i++){
        cin >> H[i] >> W[i];
        H[i]--;
        W[i]--;
        s.insert(H[i]);
        s.insert(W[i]);
        G[H[i]].push_back(W[i]);
        if(cnt[W[i]] == -1) cnt[W[i]] = 0;
        if(cnt[H[i]] == -1) cnt[H[i]] = 0;
        cnt[W[i]]++;
    }
    bool f = true;
    for(int i = 0; i < 200010; i++){
        if(cnt[i] == 0) f = false;
    }
    vector<int> d(200010, -1);
    if(f){
        d[W[0]] = 1;
    }
    for(int i = 0; i < 200010; i++){
        dfs(i, -1, G, d);
    }
    int temp = *max_element(d.begin(), d.end());
    if(f && temp == (int)s.size()) cout << s.size() << endl;
    else if(!f && temp == (int)s.size()) cout << 1 << endl;
    else cout << 0 << endl;
}