/// author: Mr.Hakimov

#include <bits/stdc++.h>
#include <chrono>

#define fi first
#define se second
#define pb push_back
#define pf push_front
#define Pb pop_back
#define Pf pop_front
#define all(x) (x).begin(), (x).end()
#define fin(s) freopen(s, "r", stdin);
#define fout(s) freopen(s, "w", stdout);

/* Just some advices:
1. If I use set/multiset, I will check it - is it correct to use set/multiset in a problem?
2. If I can't solve problem, I must write a program, which could help me to understand it much more better)
...
*/

using namespace std;

#pragma GCC optimize("Ofast")
#pragma GCC optimize ("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")

typedef long long LL;
typedef long double LD;

int TN = 1;
const int N = 2002;

vector<vector<int>> edges(N);
vector<vector<int>> edgesRev(N);

vector<bool> used(N, false);
vector<int> topSort;
vector<int> color(N, -1);

void dfsFirst(int u) {
    used[u] = true;
    for (int v : edges[u]) {
        if (!used[v]) {
            dfsFirst(v);
        }
    }
    topSort.pb(u);
}

void dfsSecond(int u, int clr) {
    color[u] = clr;
    for (int v : edgesRev[u]) {
        if (color[v] == -1) {
            dfsSecond(v, clr);
        }
    }
}

void solve() {
    int n;
    int m;
    cin >> n >> m;

    int cnt = 0;
    map<string, int> num;
    map<int, string> numRev;
    for (int i = 0; i < n; ++i) {
        string name;
        cin >> name;
        num[name] = ++cnt;
        numRev[cnt] = name;
    }

    for (int i = 0; i < m; ++i) {
        string a;
        string msr;
        string b;

        cin >> a >> msr >> b;

        int x;
        int y;
        if (a[0] == '+') {
            x = num[a.substr(1, a.size() - 1)];
        } else {
            x = num[a.substr(1, a.size() - 1)] + n;
        }

        if (b[0] == '+') {
            y = num[b.substr(1, b.size() - 1)];
        } else {
            y = num[b.substr(1, b.size() - 1)] + n;
        }

        edges[x].pb(y);
        // cout << x << " " << y << endl;
        // cout << (x > n ? x - n : x + n) << " " << (y > n ? y - n : y + n) << endl;
        edgesRev[x > n ? x - n : x + n].pb(y > n ? y - n : y + n);
        edgesRev[y].pb(x);
        edges[y > n ? y - n : y + n].pb(x > n ? x - n : x + n);
    }

    for (int i = 1; i <= 2 * n; ++i) {
        if (!used[i]) {
            dfsFirst(i);
        }
    }

    int clr = 0;
    for (int i = 2 * n - 1; i >= 0; --i) {
        if (color[topSort[i]] == -1) {
            dfsSecond(topSort[i], ++clr);
        }
    }

    for (int i = 1; i <= n; ++i) {
        if (color[i] == color[i + n]) {
            cout << -1;
            return;
        }
    }


    set<int> answer;
    for (int i = 1; i <= n; ++i) {
        if (color[i] > color[i + n]) {
            answer.insert(i);
        }

    }

    cout << answer.size() << endl;
    for (int el : answer) {
        cout << numRev[el] << endl;
    }
    cout << endl;
}

int main() {
    auto start = chrono::steady_clock::now();
    ios_base::sync_with_stdio(0);
    cin.tie(nullptr); cout.tie(nullptr);
    /// =========================================
    /// fin("input.txt"); fout("output.txt");
    /// fin("file.in"); fout("file.out");
    /// cin >> TN;
    /// =========================================
    while (TN--) solve();
    auto finish = chrono::steady_clock::now();
    auto elapsed_ms = chrono::duration_cast<chrono::milliseconds>(finish - start);
    cerr << endl << "Time: " << elapsed_ms.count() << " ms\n";
    return 0;
}