#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <tuple>
#include <cmath>
#include <numeric>
#include <functional>
#include <cassert>

#define debug_value(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << #x << "=" << x << endl;
#define debug(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << x << endl;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

using namespace std;
typedef long long ll;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    int n; cin >> n;
    vector<int> in(200000), out(200000);
    for(int i = 0; i < n; i++){
        int h, w; cin >> h >> w; h--; w--;
        out[w]++;
        in[h]++;
    }
    int cnt_s = 0;
    int cnt_t = 0;
    for(int i = 0; i < 200000; i++){
        if(abs(in[i]-out[i]) > 1){
            cout << 0 << endl;
            return 0;
        }
        if(in[i] > out[i]) cnt_t++;
        if(out[i] > in[i]) cnt_s++;
    }
    if(cnt_t ==  cnt_s){
        if(cnt_t == 0) cout << n << endl;
        else if(cnt_t == 1) cout << 1 << endl;
        else cout << 0 << endl;
    }else{
        cout<< 0 << endl;
    }
}