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

vector<pair<int, int>> pts;
vector<pair<int, int>> edges;
map<int, bool> is_point;




signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);

    int n; cin >> n;

    for(int i = 1; i <= n; i++) {
    
        int l, r; cin >> l >> r;
        edges.push_back({l, r});
    
    }

    sort(edges.begin(), edges.end());
    vector<int> edge_have(n + 1);
    int p; cin >> p;
    
    for(int i = 1; i <= p; i++) {
    
        int x; cin >> x;
        pts.push_back({x, 1});
        is_point[x] = 1;

        for(int j = 0; j < n; j++) {
            if(edges[j].first <= x and edges[j].second >= x) edge_have[j]++; 
        }
    }


    vector<pair<int, int>> calc;

    for(int i = 0; i < n - 1; i++) {
        if(edges[i].second == edges[i + 1].first) {
            calc.push_back({1, edges[i].second});
        }
    }

    for(auto x: edges) {
        int l = x.first;
        int r = x.second;
        calc.push_back({0, (l + r) / 2});
        calc.push_back({0, (l + r) / 2 + 1});
    }

    sort(calc.begin(), calc.end(), greater<pair<int, int>>());
    vector<int> res;


    for(auto x: calc) {
        if(is_point[x.second]) continue;
        int flag = 0;
        for(int i = 0; i < n; i++) {
            if(edges[i].first <= x.second and edges[i].second >= x.second) {
                if(edge_have[i] >= 2) flag = 2;
                else flag = max(flag, 1LL);
            }
        }
        
        if(flag == 1) {
            res.push_back(x.second);
            for(int i = 0; i < n; i++) {
                if(edges[i].first <= x.second and edges[i].second >= x.second) {
                    edge_have[i]++;
                }
            }
        }
    }

    for(int i = 0; i < n; i++) {
        if(edge_have[i] != 2) {
            cout << "impossible\n";
            return 0;
        }
    }

    int sz = res.size();
    cout << sz << "\n";
    for(auto x: res) cout << x << " ";
    cout << "\n";
}