#include<bits/stdc++.h>
using namespace std;
const int N=100010,M=1000010;
const int inf=0x3f3f3f3f;

int head[N],edge[M],ver[M],nxt[M],d[N];
int n,m,maxflow=0,tot=1,s=N-1,t=N-2,ss=N-3,tt=N-4;
void add_edge(int x,int y,int z)
{
	ver[++tot]=y,edge[tot]=z,nxt[tot]=head[x],head[x]=tot;
}
void add(int x,int y,int z)
{
	add_edge(x,y,z),add_edge(y,x,0);
}
void add(int x,int y,int l,int r)
{
	add(ss,y,l),add(x,tt,l),add(x,y,r-l);
}

queue<int> q;
bool bfs(int S,int T)
{
	while(q.size()) q.pop();
	memset(d,0,sizeof(d));
	q.push(S),d[S]=1;
	while(q.size())
	{
		int x=q.front(); q.pop();
		for(int i=head[x];i;i=nxt[i])
		{
			int y=ver[i];
			if(edge[i]&&!d[y]) d[y]=d[x]+1,q.push(y);
		}
	}
	return d[T];
}
int dinic(int x,int flow,int t)
{
	if(x==t||!flow) return flow;
	int res=flow,k;
	for(int i=head[x];i&&res;i=nxt[i])
	{
		int y=ver[i];
		if(edge[i]&&d[y]==d[x]+1)
		{
			k=dinic(y,min(edge[i],res),t);
			if(!k) d[y]=0;
			res-=k,edge[i]-=k,edge[i^1]+=k;
		}
	}
	return flow-res;
}

int g,l;
int minn[33],maxn[33],maxG[33];
int cnt[33],cntG[33];
bool black[33];
bool ban[33][510];
bool vis[33][510];
char st[N],col[N];
char ans[N];

int main()
{
	scanf("%d%d",&g,&l);
	for(int i=0;i<26;i++) minn[i]=0,maxn[i]=l;
		memset(ban,false,sizeof(ban));
	for(int i=1;i<g;i++)
	{
		for(int j=0;j<26;j++) cnt[j]=black[j]=0;
		scanf("%s%s",st+1,col+1);
		// cerr<<st+1<<" "<<col+1<<"\n";
		for(int j=1;j<=l;j++)
		{
			if(col[j]=='G') cnt[st[j]-'a']++,vis[st[j]-'a'][j]=true;
			if(col[j]=='Y') cnt[st[j]-'a']++,ban[st[j]-'a'][j]=true;
			if(col[j]=='B') black[st[j]-'a']=true,ban[st[j]-'a'][j]=true;
		}
		for(int j=0;j<26;j++)
		{
			minn[j]=max(minn[j],cnt[j]);
			if(black[j]) maxn[j]=min(maxn[j],cnt[j]);
		}
	}

/*
	cerr<<"minn: \n";
	for(int i=0;i<26;i++) cerr<<minn[i]<<" ";
		cerr<<"\n";
	cerr<<"maxn: \n";
	for(int i=0;i<26;i++) cerr<<maxn[i]<<" ";
		cerr<<"\n";
	cerr<<"maxG: \n";
	for(int i=0;i<26;i++) cerr<<maxG[i]<<" ";
		cerr<<"\n";
	cerr<<"ban: \n";
	for(int i=0;i<26;i++,cerr<<"\n")
	{
		cerr<<(char)(i+'a')<<": ";
		for(int j=1;j<=l;j++) cerr<<ban[i][j];
	}
	cerr<<"\n";
//*/

	for(int i=0;i<26;i++) add(s,i,minn[i],maxn[i]);

	for(int i=1;i<=l;i++)
	{
		add(26+i,t,1,1);
		bool yes=false;
		for(int ch=0;ch<26;ch++)
			if(vis[ch][i])
			{
				yes=true,add(ch,26+i,1,1);
				break;
			}
		if(yes) continue;
		for(int ch=0;ch<26;ch++)
			if(!ban[ch][i]) add(ch,26+i,0,1);
	}

/*
	for(int x=0;x<'g'-'a'+1;x++)
	{
		cerr<<(char)(x+'a')<<": \n";
		for(int i=head[x];i;i=nxt[i])
		{
			int y=ver[i];
			if(26+1<=y&&y<=26+l) cerr<<(char)(x+'a')<<" "<<y-26<<"\n";
		}
	}
//*/
/*
	cerr<<"ss = "<<ss<<", tt = "<<tt<<"\n";
	for(int i=head[ss];i;i=nxt[i])
	{
		int y=ver[i],z=edge[i];
		cerr<<"ss"<<" "<<y<<" "<<z<<"\n";
	}
	for(int i=head[tt];i;i=nxt[i])
	{
		int y=ver[i],z=edge[i];
		cerr<<"tt"<<" "<<y<<" "<<z<<"\n";
	}
//*/
/*
	for(int x=0;x<N;x++)
	{
		for(int i=head[x];i;i=nxt[i])
		{
			int y=ver[i],z=edge[i];
			if(z) cerr<<x<<" "<<y<<" "<<z<<"\n";
		}
	}
//*/
/*
	for(int i=head[t];i;i=nxt[i])
	{
		int y=ver[i];
		cerr<<t<<" "<<y<<" "<<edge[i]<<"\n";
		cerr<<y<<" "<<t<<" "<<edge[i^1]<<"\n";
	}
//*/

	add(t,s,0,inf);
	int flow=0;
	while(bfs(ss,tt)) while(flow=dinic(ss,inf,tt)) maxflow+=flow;
	
	while(bfs(s,t)) while(flow=dinic(s,inf,t)) maxflow+=flow;
	for(int i=0;i<26;i++)
	{
		for(int j=head[i];j;j=nxt[j])
		{
			int y=ver[j];
			if(26+1<=y&&y<=26+l&&!edge[j]) ans[y-26]=i+'a';
		}
	}
/*
	for(int x=0;x<N;x++)
	{
		for(int i=head[x];i;i=nxt[i])
		{
			int y=ver[i],z=edge[i];
			if(z) cerr<<x<<" "<<y<<" "<<z<<"\n";
		}
	}
//*/
	for(int i=1;i<=l;i++) putchar(ans[i]);
		putchar('\n');
	// assert(maxflow+totG==l);
	return 0;
}