///Etfsh5t nseet el rank
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx,avx2,fma")
#include "bits/stdc++.h"
using namespace std;

#define pb push_back
#define F first
#define S second
#define f(i, a, b) for (int i = a; i < b; i++)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define sz(x) (int)(x).size()
#define mp(x, y) make_pair(x, y)
#define popCnt(x) (__builtin_popcountll(x))
// #define int ll

using ll = long long;
using ull = unsigned long long;
using uint = uint32_t;
using ii = pair<int, int>;

const int N = 1e5 + 5, A = 12, LG = 18, MOD = (119 << 23) + 1;
const long double PI = acos(-1);
const long double EPS = 1e-9;
const int INF = 1e18;
string str = string(5000, '0');
map<int, set<char> > cant, can;

struct FlowEdge
{
    int v, u;
    long long cap, flow = 0;

    FlowEdge(int v, int u, long long cap) : v(v), u(u), cap(cap) {}
};

struct Dinic
{
    const long long flow_inf = 1e18;
    vector<FlowEdge> edges;
    vector<vector<int> > adj;
    int n, m = 0;
    int s, t;
    vector<int> level, ptr;
    queue<int> q;

    Dinic(int n, int s, int t) : n(n), s(s), t(t)
    {
        adj.resize(n);
        level.resize(n);
        ptr.resize(n);
    }

    void addEdge(int v, int u, long long cap)
    {
        edges.push_back(FlowEdge(v, u, cap));
        edges.push_back(FlowEdge(u, v, 0));
        adj[v].push_back(m);
        adj[u].push_back(m + 1);
        m += 2;
    }

    bool bfs()
    {
        while (!q.empty())
        {
            int v = q.front();
            q.pop();
            for (int id : adj[v])
            {
                if (edges[id].cap - edges[id].flow < 1)
                    continue;
                if (level[edges[id].u] != -1)
                    continue;
                level[edges[id].u] = level[v] + 1;
                q.push(edges[id].u);
            }
        }
        return level[t] != -1;
    }

    long long dfs(int v, long long pushed)
    {
        if (pushed == 0)
            return 0;
        if (v == t)
            return pushed;
        for (int &cid = ptr[v]; cid < (int)adj[v].size(); cid++)
        {
            int id = adj[v][cid];
            int u = edges[id].u;
            if (level[v] + 1 != level[u] || edges[id].cap - edges[id].flow < 1)
                continue;
            long long tr = dfs(u, min(pushed, edges[id].cap - edges[id].flow));
            if (tr == 0)
                continue;
            edges[id].flow += tr;
            edges[id ^ 1].flow -= tr;
            return tr;
        }
        return 0;
    }

    long long flow()
    {
        long long f = 0;
        while (true)
        {
            fill(level.begin(), level.end(), -1);
            level[s] = 0;
            q.push(s);
            if (!bfs())
                break;
            fill(ptr.begin(), ptr.end(), 0);
            while (long long pushed = dfs(s, flow_inf))
            {
                f += pushed;
            }
        }
        return f;
    }
};

void doWork()
{

    int n, l;
    cin >> n >> l;
    map<char, int> lower, upper;
    for (char c = 'a'; c <= 'z'; c++)
        upper[c] = l;
    for (int i = 1; i < n; i++)
    {
        string s, res;
        cin >> s >> res;
        map<char, int> cnt, gy;
        set<char> black;
        for (int j = 0; j < l; j++)
        {
            cnt[s[j]] += 1;
            if (res[j] == 'G')
                can[j].insert(s[j]), gy[s[j]] += 1;
            else
            {
                cant[j].insert(s[j]);
                if (res[j] == 'Y')
                {
                    gy[s[j]] += 1;
                }
                else
                {
                    black.insert(s[j]);
                }
            }
        }
        for (char c = 'a'; c <= 'z'; c++)
        {
            lower[c] = max(lower[c], gy[c]);
            if (black.count(c))
            {
                lower[c] = upper[c] = gy[c];
            }
        }
    }
    int src = l + 26;
    int snk = l + 27;
    Dinic flw(2 +l + 26, src, snk);
    for (int i = 0; i < l; i++)
    {
        flw.addEdge(src, i, 1);
        if (can.count(i))
        {
            int go = *can[i].begin() - 'a';
            flw.addEdge(i, l + go, 1);
        }
        else
        {
            for (char c = 'a'; c <= 'z'; c++)
            {
                if (cant[i].count(c))
                    continue;
                flw.addEdge(i, l + (c - 'a'), 1);
            }
        }
    }
    for (char c = 'a'; c <= 'z'; c++)
    {
        flw.addEdge(l + (c - 'a'), snk, lower[c]);
    }
    cerr << flw.flow() << endl;
    for (char c = 'a'; c <= 'z'; c++)
    {
        flw.addEdge(l + (c - 'a'), snk, upper[c] - lower[c]);
    }

    cerr << flw.flow() << endl;

    string ans = string(l, 'a');
    f(i, 0, l)
    {
        for (auto e : flw.adj[i])
        {
            if (flw.edges[e].cap == flw.edges[e].flow && flw.edges[e].u != src)
            {
                ans[i] = 'a' + (flw.edges[e].u - l);
                break;
            }
        }
    }
    cout << ans << endl;
}
int32_t main()
{
#ifdef ONLINE_JUDGE
    ios_base::sync_with_stdio(0);
    cin.tie(0);
#endif // ONLINE_JUDGE

    int t = 1;
    // cin >> t;
    while (t--)
    {
        doWork();
    }
    return 0;
}