#include<bits/stdc++.h>
#define fi first
#define se second
#define pb push_back
#define pf push_front
#define popb pop_back
#define popf pop_front
#define ins insert
#define pq priority_queue
#define minele min_element
#define maxele max_element
#define mp make_pair
#define lb lower_bound //first pos >= val
#define ub upper_bound // first pos > val
#define cnt_bit __builtin_popcount
#define all(x) x.begin(), x.end()
#define debug(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "
//#pragma GCC optimize("Ofast")
//#pragma GCC target("avx,avx2,fma")
using namespace std;

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ll> pll;
typedef pair<int, int> pii;

int d4x[4] = {1, 0, -1, 0}; int d4y[4] = {0, 1, 0, -1};
int d8x[8] = {0, 1, 1, 1, 0, -1, -1, -1};
int d8y[8] = {1, 1, 0, -1, -1, -1, 0, 1};

const int INF = 1e9;
const ll oo = 1e18;

template<class X, class Y>
    bool minimize(X &x, const Y &y) {
        if (x > y)
        {
            x = y;
            return true;
        }
        return false;
    }
template<class X, class Y>
    bool maximize(X &x, const Y &y) {
        if (x < y)
        {
            x = y;
            return true;
        }
        return false;
    }

/* Author : Le Ngoc Bao Anh, 11A5, LQD High School for Gifted Student */

map<string, int> id;
const int N = 1e5 + 10;
string name[N];
bool spec[N];
int dp[N];
vector<int> g[N];

void solve() {
    int n, k, s, m; cin >> n >> k >> s >> m;
    int cnt = 0;
    vector<string> ans;
    for(int i = 1; i <= s; i++) {
        string s; cin >> s;
        if(!id[s]) {
            id[s] = ++cnt;
            name[cnt] = s;
            ans.pb(s);
        }
        spec[id[s]] = true;
    }

    for(int i = 1; i <= m; i++) {
        string x, y;
        cin >> x >> y;
        if(!id[x]) {
            id[x] = ++cnt;
            name[cnt] = x;
        }
        if(!id[y]) {
            id[y] = ++cnt;
            name[cnt] = y;
        }
        int u = id[x], v = id[y];
        g[u].pb(v); g[v].pb(u);
    }

    for(int i = 1; i <= n; i++) {
        sort(all(g[i]));
        g[i].resize(unique(all(g[i])) - g[i].begin());
    }

    queue<int> q;
    for(int i = 1; i <= n; i++) if(spec[i]) q.push(i);
    while(!q.empty()) {
        int u = q.front(); q.pop();
        for(auto v : g[u]) {
            if(spec[v]) continue;
            ++dp[v];
            if(dp[v] >= k) {
                spec[v] = true;
                ans.pb(name[v]);
                q.push(v);
            }
        }
    }

    sort(all(ans));
    cout << ans.size() << '\n';
    for(int i = 0; i < ans.size(); i++) cout << ans[i] << " ";
}

int main()
{
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    #ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    #else
    //online
    #endif

    int tc = 1, ddd = 0;
    // cin >> tc;
    while(tc--) {
        //ddd++;
        //cout << "Case #" << ddd << ": ";
        solve();
    }
}