#include <bits/stdc++.h>
using namespace std;
const int N = 510;
namespace Flow {
    const int N = 1010;
    const int M = 1e5 + 10;
    int tot = 1;
    int a[M], c[M], ne[M], fi[N];
    inline void _Adde(int x, int y, int z) {
        a[++tot] = y; ne[tot] = fi[x]; fi[x] = tot; c[tot] = z;
    }
    inline void Adde(int x, int y, int z) {
        // printf("%d -> %d   %d\n", x, y, z);
        _Adde(x, y, z); _Adde(y, x, 0);
    }
    int cur[N], de[N];
    inline bool Bfs(int s, int t) {
        memset(de + 1, 0, t * sizeof *de);
        de[s] = 1;
        static queue<int> q;
        q.push(s);
        while (!q.empty()) {
            int u = q.front(); q.pop();
            for (int i = fi[u]; i; i = ne[i]) if (c[i] && !de[a[i]]) {
                de[a[i]] = de[u] + 1;
                q.push(a[i]);
            }
        }
        return de[t];
    }
    inline int Dfs(int x, int flow, int t) {
        if (x == t) return flow;
        int sm = 0, tmp;
        for (int &i = cur[x]; i; i = ne[i]) if (c[i] && de[a[i]] == de[x] + 1 && (tmp = Dfs(a[i], min(flow, c[i]), t))) {
            c[i] -= tmp; c[i ^ 1] += tmp;
            flow -= tmp; sm += tmp;
            if (!flow) return sm;
        }
        return sm;
    }
    inline int Solve(int s, int t) {
        int ans = 0;
        while (Bfs(s, t)) {
            memcpy(cur + 1, fi + 1, t * sizeof *cur);
            ans += Dfs(s, 1e9, t);
        }
        return ans;
    }
    inline void Print(int n) {
        for (int x = 1; x <= n; ++x) {
            for (int i = fi[x + 26]; i; i = ne[i]) if (c[i] && a[i] <= 26) {
                putchar(a[i] - 1 + 'a');
                break;
            }
        }
        puts("");
    }
}
int n, m;
int ban[N][26], U[26], L[26], E[26];
char s[N], t[N], cho[N];
int main() {
    scanf("%d%d", &n, &m); --n;
    for (int i = 0; i < 26; ++i) U[i] = 1e9, L[i] = 0;
    for (int k = 1; k <= n; ++k) {
        scanf("%s%s", s + 1, t + 1);
        static int cur[26];
        memset(cur, 0, sizeof cur);
        for (int i = 1; i <= m; ++i) {
            if (t[i] == 'G') {
                cho[i] = s[i];
                ++cur[s[i] - 'a'];
            } else if (t[i] == 'Y') {
                ban[i][s[i] - 'a'] = 1;
                ++cur[s[i] - 'a'];
            } else {
                ban[i][s[i] - 'a'] = 1;
            }
        }
        for (int i = 1; i <= m; ++i) if (t[i] == 'B') {
            U[s[i] - 'a'] = min(U[s[i] - 'a'], cur[s[i] - 'a']);
        }
        for (int i = 0; i < 26; ++i) L[i] = max(L[i], cur[i]);
    }
    // for (int i = 0; i < 26; ++i) printf("%c: %d %d\n", i + 'a', L[i], U[i]);
    int t = 26 + m + 1;
    int s1 = t++;
    int s2 = t++;
    for (int i = 1; i <= 26; ++i) {
        Flow::Adde(s1, i, L[i - 1]);
        Flow::Adde(s2, i, U[i - 1] - L[i - 1]);
    }
    for (int i = 1; i <= m; ++i) {
        Flow::Adde(i + 26, t, 1);
        if (cho[i]) Flow::Adde(cho[i] - 'a' + 1, i + 26, 1);
        else {
            for (int d = 0; d < 26; ++d) if (!ban[i][d]) Flow::Adde(d + 1, i + 26, 1);
        }
    }
    int f1 = Flow::Solve(s1, t);
    int f2 = Flow::Solve(s2, t);
    // printf("%d %d\n", f1, f2);
    Flow::Print(m);
    return 0;
}