#include <iostream>
#include <cstdio>

using namespace std;

const int inf = 1000000000;

struct E {
	int to, nxt, w;
}e[100005];

int g, l, nlk[233][505];
int n, s, t, u, v, w, cnt, Ans, h[666], cur[666], dis[666], q[666];

char s1[505][505], s2[505][505], ans[505];
int vis[233], mnvis[233], mxvis[233], vans[505], anscnt[233], visB[233];

void add(int u, int v, int w) {
	e[++cnt] = (E){v, h[u], w}, h[u] = cnt;
}
int bfs() {
	for(int i = 1; i <= n; ++i) {
		dis[i] = inf;
	}
	dis[s] = 0, q[1] = s;
	int hd = 0, tl = 1;
	while(hd < tl) {
		int u = q[++hd];
		for(int i = h[u]; i; i = e[i].nxt) {
			int v = e[i].to;
			if(e[i].w > 0 && dis[v] == inf) {
				q[++tl] = v, dis[v] = dis[u] + 1;
				if(v == t) return 1;
			}
		}
	}
	return 0;
}
int dfs(int u, int flow) {
	if(u == t) return flow;
	int res = 0;
	for(int i = cur[u]; i; i = e[i].nxt) {
		int v = e[i].to;
		cur[u] = i;
		if(e[i].w > 0 && dis[v] == dis[u] + 1) {
			int F = dfs(v, min(flow, e[i].w));
			if(!F) dis[v] = inf;
			e[i].w -= F, e[i ^ 1].w += F, res += F, flow -= F;
			if(!flow) break;
		} 
	}
	return res;
}

int main() {
	scanf("%d%d", &g, &l);
	for(int i = 1; i < g; ++i) {
		scanf("%s %s", s1[i] + 1, s2[i] + 1);
		for(int j = 1; j <= l; ++j) {
			if(s2[i][j] == 'G') {
				vans[j] = 1, ans[j] = s1[i][j];
			}
		}
	}
	for(int i = 1; i <= l; ++i) if(vans[i]) {
		++anscnt[ans[i]];
	}
	for(int c = 'a'; c <= 'z'; ++c) {
		mxvis[c] = l;
	}
	for(int i = 1; i < g; ++i) {
		for(int c = 'a'; c <= 'z'; ++c) {
			vis[c] = visB[c] = 0;
		}
		for(int j = 1; j <= l; ++j) {
			if(s2[i][j] == 'Y') {
				nlk[s1[i][j]][j] = 1;
				++vis[s1[i][j]];
			}
			else if(s2[i][j] == 'G') {
				++vis[s1[i][j]];
			}
			else {
				nlk[s1[i][j]][j] = 1;
				visB[s1[i][j]] = 1;
			}
			// if(s2[i][j] == 'G') {
			// 	continue;
			// }
			// else {
			// 	++vis[s1[i][j]];
			// 	if(s2[i][j] == 'Y') {
			// 		nlk[s1[i][j]][j] = 1;
			// 	}
			// }
		}
		for(int c = 'a'; c <= 'z'; ++c) {
			mnvis[c] = max(mnvis[c], vis[c] - anscnt[c]);
			if(visB[c]) {
				mxvis[c] = min(mxvis[c], vis[c] - anscnt[c]);
			}
		}
	}
	cnt = 1;
	s = 26 + l + 1, t = 26 + l + 2;
	n = t;
	for(int i = 1; i <= 26; ++i) {
		add(s, i, mnvis['a' + i - 1]), add(i, s, 0);
		for(int j = 1; j <= l; ++j) if(!nlk['a' + i - 1][j] && !vans[j]) {
			add(i, 26 + j, 1), add(26 + j, i, 0);
		}
	}
	for(int i = 1; i <= l; ++i) if(!vans[i]) {
		add(26 + i, t, 1), add(t, 26 + i, 0);
	}
	while(bfs()) {
		for(int i = 1; i <= n; ++i) {
			cur[i] = h[i];
		}
		Ans += dfs(s, inf);
	}

	for(int i = 1; i <= 26; ++i) {
		for(int j = h[i]; j; j = e[j].nxt) {
			if(e[j].to <= 26 + l && e[j].w < 1) {
				ans[e[j].to - 26] = 'a' + i - 1;
				vans[e[j].to - 26] = 1;
			}
		}
	}

	for(int i = 1; i <= 26; ++i) {
		// printf("%c %d %d\n", 'a' + i - 1, mxvis['a' + i - 1], mnvis['a' + i - 1]);
		add(s, i, mxvis['a' + i - 1] - mnvis['a' + i - 1]), add(i, s, 0);
	}
	for(int i = 1; i <= l; ++i) if(vans[i]) {
		for(int j = h[26 + i]; j; j = e[j].nxt) {
			e[j].w = e[j ^ 1].w = 0;
		}
	}
	while(bfs()) {
		for(int i = 1; i <= n; ++i) {
			cur[i] = h[i];
		}
		Ans += dfs(s, inf);
	}

	for(int i = 1; i <= 26; ++i) {
		for(int j = h[i]; j; j = e[j].nxt) {
			if(e[j].to <= 26 + l && !vans[e[j].to - 26] && e[j].w < 1) {
				ans[e[j].to - 26] = 'a' + i - 1;
				vans[e[j].to - 26] = 1;
			}
		}
	}
	for(int i = 1; i <= l; ++i) {
		putchar(ans[i]);
	}
	return 0;
}