#include<bits/stdc++.h>
using namespace std;

// 사용법

// Dinic dinic(n, src, snk) : 정점 n개 (0-based)인 플로우 그래프를 만든다.
// 이 때, 소스는 src, 싱크는 snk로 설정할것이다.

// dinic.AddEdge(a, b, c) : a -> b 방향으로 용량 c인 간선을 생성한다.

// dinic.AddLR(a, b, l, r) : a -> b 방향으로 유량에 [l,r] 제약이 달린 간선을 생성한다.

// dinic.check_circulation() : LR제약을 모두 만족하는 flow가 존재한다면 true를 리턴한다.

struct Dinic {
	struct Edge { int next, rev, c, f; };
	int N, src, snk, SRC, SNK;
	int need;
	vector<vector<Edge> > g;
	vector<int> dist, p, pp;
	void AddEdge(int a, int b, int c) {
		g[a].push_back({b, g[b].size(), c, 0});
		g[b].push_back({a, g[a].size()-1, 0, 0});
	}
	Dinic() { }
	Dinic(int n, int s, int t) {
		N = n;
		src = s; snk = t;
		SRC = N; SNK = N+1; N += 2;
		need = 0;
		g.resize(N);
		AddEdge(snk, src, 1234567890);
	}
	void AddLR(int a, int b, int l, int r) { // [l, r]
		assert(l <= r);
		AddEdge(a, b, r-l);
		AddEdge(a, SNK, l);
		AddEdge(SRC, b, l);
		need += l;
	}
	int Run(int src, int snk) {
		int ret = 0;
		while(1)
		{
			dist = p = pp = vector<int>(N, -1);
			queue<int> q;
			q.push(src), dist[src] = 0;
			while(!q.empty())
			{
				int n = q.front(); q.pop();
				for(int i=0; i<g[n].size(); i++)
				{
					auto &it = g[n][i];
					if (dist[it.next] == -1 && it.c - it.f > 0)
					{
						q.push(it.next);
						dist[it.next] = dist[n] + 1;
						p[it.next] = n;
						pp[it.next] = i;
					}
				}
			}
			if (dist[snk] == -1) break;

			int flow = 1234567890;
			for(int i=snk; i!=src; i=p[i])
				flow = min(flow, g[p[i]][pp[i]].c - g[p[i]][pp[i]].f);
			for(int i=snk; i!=src; i=p[i])
			{
				g[p[i]][pp[i]].f += flow;
				g[i][g[p[i]][pp[i]].rev].f -= flow;
			}
			ret += flow;
		}
		return ret;
	}
	bool check_circulation()
	{
		int flow = Run(SRC, SNK);
		Run(src, snk);
		if (flow == need) return true;
		else return false;
	}
};

int n, m, L[256], R[256], FL[256], FR[256], ans[555];
bool chk[555][555];
string s[555], t[555];

int main()
{
	ios::sync_with_stdio(0); cin.tie(0);
	cin >> m >> n; m--;
	for(int i=1;i<=m;i++) cin >> s[i] >> t[i];

	for(int i='a';i<='z';i++) FL[i] = 0, FR[i] = n;

	for(int i=1;i<=m;i++) {
		memset(L, 0, sizeof L);
		memset(R, -1, sizeof R);

		for(int j=0;j<n;j++) {
			if(t[i][j] == 'G') {
				ans[j] = s[i][j];
				L[s[i][j]]++;
			}
		}

		for(int j=0;j<n;j++) {
			if(t[i][j] == 'Y') {
				L[s[i][j]]++;
				chk[j][s[i][j]] = 1;
			}
			if(t[i][j] == 'B') {
				R[s[i][j]] = L[s[i][j]];
				chk[j][s[i][j]] = 1;
			}
		}

		for(int i='a';i<='z';i++) {
			FL[i] = max(FL[i], L[i]);
			if(R[i] != -1) FR[i] = min(FR[i], R[i]);
		}
	}

	for(int i=0;i<n;i++) if(ans[i]) {
		FL[ans[i]]--, FR[ans[i]]--;
	}

	int src = 0, snk = n + 26 + 1;
	Dinic dinic(n + 28, src, snk);

	vector<vector<int> > p(n + 1, vector<int>(26, -1));
	for(int i=0;i<n;i++) if(!ans[i]) {
		dinic.AddLR(src, i+1, 1, 1);
		for(int j='a';j<='z';j++) if(!chk[i][j]) {
			p[i+1][j-'a'] = dinic.g[i+1].size();
			dinic.AddEdge(i+1, n+j-'a'+1, 1);
		}
	}

	for(int i='a';i<='z';i++) {
		dinic.AddLR(n+i-'a'+1, snk, FL[i], FR[i]);
	}

	bool tmp = dinic.check_circulation();
	assert(tmp == true);

	vector<pair<int, int> > e;
	for(int i=1; i<=n; i++)
		for(int j=0; j<26; j++)
			if (p[i][j] != -1 && dinic.g[i][p[i][j]].f == 1)
				e.push_back({i, j});

	for(auto [a,b] : e)
		ans[a-1] = (char)(b+'a');

	for(int i=0;i<n;i++) cout << char(ans[i]);
	cout << endl;
}