#include <bits/stdc++.h>
using namespace std;
struct UnionFind{
    vector<int> par;
    vector<int> rank;
    UnionFind(int n){
        par.resize(n);
        rank.resize(n);
        for(int i = 0; i < n; i++){
            par[i] = i;
            rank[i] = 1;
        }
    }

    int root(int x){
        if(par[x] == x) return x;
        else return par[x] = root(par[x]);
    }

    bool same(int x, int y){
        return root(x) == root(y);
    }

    void unite(int x, int y){
        x = root(x);
        y = root(y);
        if(x == y) return;
        if(rank[x] < rank[y]) swap(x, y);
        par[y] = x;
        rank[x] += rank[y];
    }

    int size(int x) {
        return rank[root(x)];
    }
};
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 || t == par) continue;
        d[now] = max(d[now], dfs(t, now, G, d));
    }
    d[now]++;
    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);
    UnionFind U(200010);
    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;
        U.unite(W[i], H[i]);
        cnt[W[i]]++;
    }
    bool f = true;
    vector<int> d(200010, -1);
    for(int i = 0; i < 200010; i++){
        if(cnt[i] == 0) {
            f = false;
        }
    }
    int temp = 0;
    for(int i = 0; i < 200010; i++){
        temp = max(U.size(i), temp);
        dfs(i, -1, G, d);
    }
    if(f) {
        if(temp == (int)s.size()) cout << s.size() << endl;
        else cout << 0 << endl;
    }
    else {
        temp = *max_element(d.begin(), d.end());
        if(temp == (int)s.size()) cout << 1 << endl;
        else cout << 0 << endl;
    }

}