#include <bits/stdc++.h>

typedef long long ll;

using namespace std;

const int inf = INT_MAX;

struct maximum_bipartite_matching { ;
    int nl, nr, mbm = 0;
    vector<bool> v;
    vector<int> ml, mr, d;
    vector<vector<int>> &e;

    void fnc(bool bul = false) {
        int prv = mbm;
        do {
            prv = mbm;
            d.assign(nr, inf);
            queue<int> q;
            for(int i = 0;i < nl; i++)
                if(ml[i] == -1)
                    for(int j : e[i])
                        if(d[j] == inf)
                            d[j] = 0, q.push(j);
            while(!q.empty() && !bul) {
                int i = q.front(); q.pop();
                if(mr[i] == -1) {
                    for(int j = 0; j < nr; j++)
                        if(d[j] > d[i])
                            d[j] = inf;
                    break;
                }
                for(int j : e[mr[i]])
                    if(d[j] == inf)
                        d[j] = d[i] + 1, q.push(j);
            }
            for(int i = 0; i < nl; i++)
                if(ml[i] == -1)
                    mbm += findPath(i);
        } while(mbm > prv);
    }

    maximum_bipartite_matching(int nl, int nr, vector<vector<int>> &e): nl(nl), nr(nr), ml(nl, -1), mr(nr, -1), e(e) {
        fnc();
    }

    void update() {
        int old = nl;
        nl = e.size();
        ml.resize(nl);
        for(int i = old; i < nl; i++)
            ml[i] = -1;
        fnc(true);
    }

    bool findPath(int i, int l = 0) {
        for(int j : e[i])
            if(d[j] == l) {
                d[j] = inf;
                if(mr[j] == -1 || findPath(mr[j], l + 1)) {
                    ml[i] = j; mr[j] = i;
                    return true;
                }
            }
        return false;
    }
};


int main() {
#ifdef WITHFILE
    freopen("1.in", "r", stdin);
    // freopen("1.out", "w", stdout);
#endif

    cin.sync_with_stdio(false);
    cin.tie(0);

    int N, L;
    cin >> N >> L;
    string ans(L, 0);
    vector< map<char, bool> > no(L, map<char, bool>());
    map<char, int> cntmin, fxt, nrmax;
    map<char, bool> nono;
    for(int q = 1; q <= N - 1; q++) {
        string a, b;
        cin >> a >> b;
        map<char, int> fq;
        for(int i = 0; i < L; i++) {
            if(b[i] == 'G') {
                ans[i] = a[i];

            } else {
                no[i][a[i]] = true;
            }
            if(b[i] == 'G' || b[i] == 'Y')
                fq[ a[i] ]++;
        }

        for(int i = 0; i < L; i++) {
            if(b[i] == 'B' && fq[ a[i] ] == 0) {
                nono[ a[i] ] = true;
            }
            if(b[i] == 'B' && fq[ a[i] ] != 0) {
                nrmax[ a[i] ] = fq[ a[i] ];
            }
        }

        for(auto [ch, x]: fq)
            cntmin[ch] = max(cntmin[ch], x);
    }

    for(int i = 0; i < L; i++)
        if(ans[i] != 0)
            fxt[ ans[i] ]++;

    vector<vector<int>> ee;
    vector<char> wh;
    for(auto [ch, x]: cntmin) {
        int l = cntmin[ch] - fxt[ch];
        for(int i = 0; i < l; i++) {
            wh.push_back(ch);
            vector<int> e;
            for(int j = 0; j < L; j++) {
                if(!no[j][ch]  && ans[j] == 0) {
                    e.push_back(j);
                }
            }
            ee.push_back(e);
        }
    }

    maximum_bipartite_matching mtc(ee.size(), L, ee);

    int kk = ee.size();

    for(int i = 0; i < ee.size(); i++)
        ans[ mtc.ml[i] ] = wh[i];

    int nw = kk;
    vector<int> where[1124];
    for(int i = 0; i < L; i++) {
        for(char j = 'a'; j <= 'z'; j++) {
            wh.push_back(j);
            where[j].push_back((int)wh.size() - 1);
            ee.push_back(vector<int>());
        }
    }

    for(int i = 0; i < L; i++)
        if(ans[i] == 0) {
            for(char a = 'a'; a <= 'z'; a++) {
                if(no[i][a])    continue;
                if(nono[a]) continue;
                if(nrmax.count(a)) continue;
                for(auto p: where[a])
                    ee[p].push_back(i);
            }
        }

    mtc.e = ee;
    mtc.update();

    for(int i = 0; i < ee.size(); i++)
        if(mtc.ml[i] != -1)
            ans[ mtc.ml[i] ] = wh[i];

    cout << ans << '\n';

    return 0;
}