#include<bits/stdc++.h>
#define inf 1e9
#define eps 1e - 6
#define FOR(i, a, b) for(int i = a;i <= b;i ++)
#define REP(i, a, b) for(int i = a;i >= b;i --)
#define pa pair<int, int>
#define fr first
#define sd second
#define pb push_back
#define db double
#define MEM(a) memset(a, 0, sizeof(a))
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
inline ll read()
{
	char ch = getchar();
	ll s = 0, w = 1;
	while (ch < '0' || ch > '9') {if (ch == '-')w = -1; ch = getchar();}
	while (ch >= '0' && ch <= '9') {s = s * 10 + ch - '0'; ch = getchar();}
	return s * w;
}
int n, m;
#define N 610
#define M 200010
struct edge
{
	int nxt, to, fl;
} e[M << 1];
int h[N], cur[N], cnt = 1;
int ans[N], L[N], R[N];
int g[N][27];
char a[N], b[N];
int S, T, SS, TT;
int in[N], out[N];
int dep[N];
inline void add(int x, int y, int fl)
{
	e[++cnt] = (edge) {h[x], y, fl}; h[x] = cnt;
	e[++cnt] = (edge) {h[y], x, +0}; h[y] = cnt;
}
inline void add(int x, int y, int d, int u)
{
	add(x, y, u - d);
	out[x] += d; in[y] += d;
}
int bfs()
{
	queue<int>Q; FOR(i, 1, T)dep[i] = 0, cur[i] = h[i];
	dep[S] = 1; Q.push(S);
	while (!Q.empty())
	{
		int x = Q.front(); Q.pop();
		for (int i = h[x]; i; i = e[i].nxt)if (e[i].fl && !dep[e[i].to])
			{
				int y = e[i].to; Q.push(y); dep[y] = dep[x] + 1;
				if (y == T)return 1;
			}
	}
	return 0;
}
int dfs(int x, int flow)
{
	if (x == T)return flow;
	int res = 0;
	for (int &i = cur[x]; i; i = e[i].nxt)if (e[i].fl && dep[e[i].to] == dep[x] + 1)
		{
			int y = e[i].to, fl = dfs(y, min(flow - res, e[i].fl));
			e[i].fl -= fl;
			e[i ^ 1].fl += fl;
			res += fl;
			if (res == flow)return res;
		}
	if (!res) dep[x] = 0;
	return res;
}
void Dinic()
{
	while (bfs()) {dfs(S, inf);}
}
int main()
{
	n = read() - 1, m = read();
	FOR(i, 1, 26) R[i] = m;
	FOR(i, 1, n)
	{
		scanf("%s%s", a + 1, b + 1);
		vector<int> s(27, 0), vst(27, 0);
		FOR(j, 1, m)
		{
			int t = a[j] - 'a' + 1;
			if (b[j] == 'G')
			{
				s[t]++; ans[j] = t;
			}
			else if (b[j] == 'Y') s[t] ++, g[j][t] = 1;
			else
			{
				g[j][t] = 1;
				vst[t] = 1;
			}
		}
		FOR(j, 1, 26) L[j] = max(L[j], s[j]);
		FOR(j, 1, 26)if (vst[j])R[j] = L[j];
	}
	SS = m + 27, TT = SS + 1, S = TT + 1, T = S + 1;
	// FOR(i, 1, 26)cerr << L[i] << ' ' << R[i] << '\n';
	FOR(i, 1, m)if (ans[i])L[ans[i]]--, R[ans[i]]--;
	FOR(i, 1, 26)add(SS, i, L[i], R[i]);
	FOR(i, 1, m)if (!ans[i])add(i + 26, TT, 1, 1);
	FOR(j, 1, 26)FOR(i, 1, m)if (!g[i][j])add(j, i + 26, 0, 1);
	add(TT, SS, 0, inf);
	FOR(i, 1, TT)
	{
		if (in[i] > out[i])add(S, i, in[i] - out[i]);
		else add(i, T, out[i] - in[i]);
	}
	Dinic();
	FOR(i, 1, m)
	{
		if (ans[i]) {cout << char(ans[i] + 'a' - 1); continue;}
		for (int j = h[i + 26]; j; j = e[j].nxt)
		{
			int y = e[j].to;
			if (e[j].fl && y <= 26) {cout << char('a' + y - 1); }
		}
	}
	return 0;
}