#include <bits/stdc++.h>
using namespace std;
 
typedef long long ll;
typedef long double ld;
typedef unsigned int uint;
typedef vector<int> vi;
typedef vector< vector <int> > vvi;
typedef pair<int, int> pii;
typedef pair < pair < int, int >, int > piii;
typedef pair < pair <int, int > , pair <int, int> > piiii;
typedef pair<ll, ll> pll;
typedef vector<bool> vb;
typedef vector<char> vc;
typedef vector<string> vs;
 
#define FOR(i,a,b) for(int i = a; i < b; i ++)
#define RFOR(i,a,b) for(int i = a-1; i >= b; i --)
#define all(a) a.begin(), a.end()
//#define endl '\n';
#define sz(x) (int)(x).size()
 
#define mp make_pair
#define pb push_back
#define ff first
#define ss second
 
template <typename T>
void pr(vector<T> &v) {
    FOR(i, 0, sz(v)) cout << v[i] << " ";
    cout << endl;
}
template <typename T>
void pr(vector<vector<T> > &v) {
    FOR(i, 0, sz(v)) { pr(v[i]); }
}
template <typename T>
void re(T &x) {
    cin >> x;
}
template <typename T>
void re(vector<T> &a) {
    FOR(i, 0, sz(a)) re(a[i]);
}
template <class Arg, class... Args>
void re(Arg &first, Args &... rest) {
    re(first);
    re(rest...);
}
template <typename T>
void pr(T x) {
    cout << x << endl;
}
template <class Arg, class... Args>
void pr(const Arg &first, const Args &... rest) {
    cout << first << " ";
    pr(rest...);
    cout << endl;
}
void ps() { cout << endl; }
template<class T, class... Ts>
void ps(const T& t, const Ts&... ts) {
    cout << t; if (sizeof...(ts)) cout << " "; ps(ts...);
}
 
const ll MOD  =  998244353;
#define inf 1e18;
#define INF INT_MAX;
 
long double PI = 4*atan(1);
long double eps = 1e-12;

bool dfs(int a, int L, vector<vi>& g, vi& btoa, vi& A, vi& B) {
    if (A[a] != L) return 0;
    A[a] = -1;
    for (int b : g[a]) if (B[b] == L + 1) {
        B[b] = 0;
        if (btoa[b] == -1 || dfs(btoa[b], L + 1, g, btoa, A, B)) {
            return btoa[b] = a, 1;
        }
    }
    return 0;
}

int hopcroftKarp(vector<vi>& g, vi& btoa) {
    int res = 0;
    vi A(g.size()), B(btoa.size()), cur, next;
    for (;;) {
        fill(all(A), 0);
        fill(all(B), 0);
        cur.clear();
        for (int a : btoa) if(a != -1) A[a] = -1;
        FOR(a,0,sz(g)) if(A[a] == 0) cur.push_back(a);

        for (int lay = 1;; lay++) {
            bool islast = 0;
            next.clear();
            for (int a : cur) for (int b : g[a]) {
                if (btoa[b] == -1) {
                    B[b] = lay;
                    islast = 1;
                }
                else if (btoa[b] != a && !B[b]) {
                    B[b] = lay;
                    next.push_back(btoa[b]);
                }
            }
            if (islast) break;
            if (next.empty()) return res;
            for (int a : next) A[a] = lay;
            cur.swap(next);
        }

        FOR(a,0,sz(g)) res += dfs(a, 0, g, btoa, A, B);
    }
}


vi upper(26, 1e9);
vi lower(26, 0);
int green[26] = {};
int pos_to_orig[510] = {};
char idxtochar[510] = {};
int banned[510][26] = {};
vvi adj;



int main() {
    //auto start = chrono::high_resolution_clock::now();
    ios_base::sync_with_stdio(0);cin.tie(0);
    // freopen("promote.in", "r", stdin);
	// freopen("promote.out", "w", stdout);
    #ifdef DEBUG
      freopen("input.txt", "r", stdin);
      freopen("output.txt", "w", stdout);
    #endif 

    int g,L; cin >> g >> L;
    vector<string> guess(g-1), res(g-1);

    FOR(i,0,g-1) {
        cin >> guess[i] >> res[i];
    }

    string ans = "";
    FOR(i,0,L) ans += '*';

    FOR(i,0,g-1) {
        int cnt[26][3] = {};
        FOR(j,0,L) {
            if(res[i][j] == 'G') {
                cnt[guess[i][j]-'a'][2]++;
                ans[j] = guess[i][j];
            }
            else if(res[i][j] == 'Y') {
                cnt[guess[i][j]-'a'][1]++;
                banned[j][guess[i][j] - 'a'] = 1;
            } else {
                cnt[guess[i][j]-'a'][0]++;
                banned[j][guess[i][j] - 'a'] = 1;
            }
        }

        FOR(j,0,26) {
            lower[j] = max(lower[j], cnt[j][1] + cnt[j][2]);
            if(cnt[j][0] > 0) {
                int maxv = cnt[j][1] + cnt[j][2];
                upper[j] = min(upper[j], maxv);
            }
        }
    }

  //  cout << ans << endl;
    int num_need_ans = 0;
    FOR(i,0,L) {
        if(ans[i] == '*') {
            pos_to_orig[num_need_ans] = i;
            num_need_ans++;
        } else {
            green[ans[i] - 'a']++;
        }
    }

    int num_need_char = 0;
    FOR(i,0,26) {
        int rem = min(lower[i] - green[i], upper[i] - green[i]);
        FOR(j,0,rem) {
            idxtochar[num_need_char] = 'a' + i;
            num_need_char++;
        }
    }

    if(num_need_char < num_need_ans) {
        char newchar = 'a';
        FOR(i,0,26) if(upper[i] > 1e6) {newchar = 'a' + i; break;}
        while(num_need_char < num_need_ans) {
            idxtochar[num_need_char] = newchar;
            num_need_char++;
        }
    }

    int nn = num_need_ans;
    adj.resize(nn);
    

    FOR(i,0,nn) {
        int o_idx = pos_to_orig[i];
        FOR(j,0,nn) {
            char res = idxtochar[j];
            if(banned[o_idx][res-'a'] == 0) {
                adj[i].pb(j);
            }
        }
    }

    vi btoa(nn, -1);
    int tot = hopcroftKarp(adj, btoa);


    FOR(i,0,nn) {
        int idx = btoa[i];
        ans[pos_to_orig[idx]] = idxtochar[i];
    }

    cout << ans << endl;





















    


    // auto stop = chrono::high_resolution_clock::now();
    // auto duration = chrono::duration_cast<chrono::microseconds>(stop - start);
    // cout << duration.count() << endl;
    //cin.close();
    //cout.close();
}