#include <bits/stdc++.h>
using namespace std;

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

    vector<pair<string, string>>allatok(n);
    set<string>kaja;
    set<string> out;


    for (int i =0; i < n ;i++) {
        cin >> allatok[i].first;
        //cout<<"s";
        cin >> allatok[i].second;
        kaja.insert(allatok[i].first);
    }

    for (int i = 0; i < n;i++) {
        if (kaja.count(allatok[i].second)) {
            //cout<<endl<<allatok[i].second<< " is in the set: "<<kaja.count(allatok[i].second);
            out.insert(allatok[i].first);
        }
    }

    cout<<out.size()<<endl;
    for (string x : out) {
        cout<<x<<endl;
    }




}