#include <bits/stdc++.h>
using namespace std;
#define _ <<" "<<
#define sz(x) ((int) (x).size())
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<vector<int>> Graph;
typedef vector<vector<pair<int,ll>>> Graphl;
const int maxn = 5e5 + 5;
struct Dinic {

    typedef long long T;
    T oo = INT_MAX;

    struct Edge {
        int v;
        T c;
    };
    int n, s, t;
    vector<int> dep, cur;
    vector<Edge> edges;
    vector<vector<int>> G;

    Dinic(int n) : n(n), cur(n + 1), G(n + 1) {}

    int addedge(int x, int y, T c) {
        G[x].push_back(sz(edges));
        edges.push_back({y, c});
        G[y].push_back(sz(edges));
        edges.push_back({x, 0});
        return sz(edges) - 2;
    }

    bool bfs() {
        dep.assign(n + 1, 0);
        queue<int> q;
        q.push(s);
        dep[s] = 1;

        while (!q.empty()) {
            int x = q.front();
            q.pop();

            for (auto i : G[x]) {
                auto [v, c] = edges[i];

                if (abs(c) > 1e-9 && !dep[v]) {
                    dep[v] = dep[x] + 1;
                    q.push(v);
                }
            }
        }

        return dep[t];
    }

    T dfs(int x, T flow) {
        if (x == t)
            return flow;

        T rest = flow;

        for (int &i = cur[x]; i < sz(G[x]); ++i) {
            auto &[v, c] = edges[G[x][i]];

            if (abs(c) > 1e-9 && dep[v] == dep[x] + 1) {
                T k = dfs(v, min(c, rest));

                if (abs(k) < 1e-9)
                    dep[v] = 0;

                rest -= k;
                c -= k;
                edges[G[x][i] ^ 1].c += k;
            }
        }

        return flow - rest;
    }

    T maxflow(int ts, int tt) {
        s = ts;
        t = tt;
        T ret = 0, flow;

        while (bfs()) {
            cur.assign(n + 1, 0);

            while ((flow = dfs(s, oo)))
                ret += flow;
        }

        return ret;
    }
};

int n, m;

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cin >> m >> n;
    vector<char> ans(n+1);
    vector<int> L(26),R(26);
    for (int i = 0;i < 26; ++i) R[i] = n;
    // int fixed = 0;
    vector<vector<int>> noedge(n+1, vector<int>(26));
    for (int i = 1;i <= m; ++i) {
        string a, b;
        cin >> a >> b;
        vector<int> l(26), cnt(26), isblack(26);
        for (int j = 0;j < sz(b); ++j) {
            if (b[j] == 'G') ans[j+1] = a[j], l[a[j]-'a']++;
            else if (b[j] == 'B') isblack[a[j]-'a'] = 1, noedge[j+1][a[j]-'a'] = 1;
            else l[a[j]-'a']++,noedge[j+1][a[j]-'a'] = 1;
        }
        for (int c = 0;c < 26; ++c) {
            if (isblack[c]) {
                L[c] = l[c]; R[c] = l[c];
            } else {
                L[c] = max(L[c], l[c]);
            }
        }
    }
    // for (int c = 0;c < 26; ++c) {
    //     cout << (char)(c+'a') _ L[c] _ R[c] << endl;
    // }
    Dinic dinic(n + 26 + 4);
    int s = n+26 + 1; int t = n+26+2;
    int ss = s + 2; int tt = t + 2;
    vector<vector<int>> id(n+1, vector<int>(26));
    for (int i = 1;i <= n; ++i) {
        dinic.addedge(s,i,1);
        if (ans[i]) {
            id[i][ans[i]-'a'] = dinic.addedge(i,n+ans[i]-'a'+1, 1);
            
            // cout << i _ ans[i] _ "id" _ id[i][ans[i]-'a']<< endl;
            continue;
        }
        for (char c = 0;c < 26; ++c) if(!noedge[i][c]) {
            id[i][c] = dinic.addedge(i, n+c+1, 1);
        }
    }
    int outflow = 0;
    vector<int> adde;
    for (int i = n+1;i <= n+26; ++i) {
        int c = i - n - 1;
        dinic.addedge(i, t, R[c] - L[c]);
        outflow += L[c];
        adde.push_back(dinic.addedge(i,tt, L[c]));    
    }
    adde.push_back(dinic.addedge(ss, t, outflow));
    adde.push_back(dinic.addedge(t, s, n));

    dinic.maxflow(ss, tt);
    for (auto e : adde) {
        dinic.edges[e].c = dinic.edges[e^1].c = 0;
    }
    dinic.maxflow(s, t);
    for (int i = 1;i <= n; ++i) {
        for (int c = 0;c < 26; ++c) if (id[i][c]) {
            if (dinic.edges[id[i][c]^1].c != 0) {
                // cout << i _ (char)(c+'a') _ c << endl;
                ans[i] = c;
            }
        }
    }
    for (int i = 1;i <= n; ++i) {
        cout << (char)(ans[i] + 'a');
    }
    cout << '\n';
    return 0;
}