#include<bits/stdc++.h>
using namespace std;
#define mpr make_pair
#define pb push_back
#define fi first
#define se second

const int maxn=555;
int g,l;
string wd[maxn],fdb[maxn];
int lim[maxn],ty[maxn];//0:>=,1=
int ban[26][maxn];
char sure[maxn];
signed main(){
    ios::sync_with_stdio(0);
    cin.tie(0),cout.tie(0);
    cin>>g>>l;
    g--;
    for(int i=1;i<=g;i++){
        cin>>wd[i]>>fdb[i];
        vector<int>cnt(26),bk(26);
        for(int j=0;j<l;j++){
            if(fdb[i][j]=='G'){
                sure[j]=wd[i][j];
                cnt[wd[i][j]-'a']++;
            }
        }
        for(int j=0;j<l;j++){
            ban[wd[i][j]-'a'][j]=1;
            if(fdb[i][j]=='Y'){
                cnt[wd[i][j]-'a']++;
            }
            if(fdb[i][j]=='B'){
                ty[wd[i][j]-'a']=1;
                lim[wd[i][j]-'a']=cnt[wd[i][j]-'a'];
                bk[wd[i][j]-'a']=1;
            }
        }
        for(int j=0;j<26;j++){
            if(bk[j]==0){
                if(ty[j]==0){
                    lim[j]=max(lim[j],cnt[j]);
                }
            }
        }
    }
    for(int i=0;i<l;i++){
        if(sure[i])
        lim[sure[i]-'a']--;
    }
    for(int i=0;i<26;i++){
        for(int j=0;j<l;j++){
            if(ban[i][j]==0&&lim[i]&&sure[j]==0){
                sure[j]=i+'a';
                lim[i]--;
            }
        }
    }
    assert(*max_element(lim,lim+26)==0);
    for(int i=0;i<26;i++){
        for(int j=0;j<l;j++){
            if(sure[j]==0&&ban[i][j]==0&&ty[i]==0){
                sure[j]=i+'a';
            }
        }
    }
    cout<<sure<<"\n";
    return 0;
}