#include <stdio.h>
#include <string.h>

#define MAXNODES 256

FILE *infile;
int nodes, start, ende;

int used[MAXNODES], startused[MAXNODES];
int queue[MAXNODES], queue_in, queue_out;
unsigned char g[MAXNODES][MAXNODES][256];
unsigned char reach[MAXNODES][MAXNODES][256];

void queue_init() { queue_in = queue_out = 0; }
int queue_empty() { if (queue_in == queue_out) return 1; return 0; }
void enqueue(int c) { queue[queue_in] = c; queue_in++; }
int dequeue() { int c; c = queue[queue_out]; queue_out++; return c; }


int read_case()
{
	int i, j, k, a, b;
	char comps[32];
	fscanf(infile, "%d", &nodes);
	if (nodes == 0) return 0;
	for (i=0; i<MAXNODES; i++) for (j=0; j<=MAXNODES; j++) for (k='a'; k<='z'; k++)
	{
		g[i][j][k] = 0;
		reach[i][j][k] = 0;
	}
	fscanf(infile, "%d %d", &a, &b);
	while ((a!=0) && (b!=0))
	{
		fscanf(infile, "%s", comps);
		for (j=0; j<strlen(comps); j++) g[a][b][(int)comps[j]] = 1;		
		fscanf(infile, "%d %d", &a, &b);
	}
	return 1;
}



int search(int c)
{
	int i, j, k, d;
	for (i=0; i<MAXNODES; i++) used[i] = 0;
	queue_init();
	enqueue(start);
	used[start] = 1;
	while (queue_empty()==0)
	{
		d = dequeue();
		reach[start][d][c] = 1;
		for (i=1; i<=nodes; i++)
		{
			if ((g[d][i][c] == 1) && (used[i]==0))
			{
				used[i] = 1;
				enqueue(i);
			}
		}
	}
	return 0;

}

void solve_case()
{
	int c, sols, i;
	fscanf(infile, "%d %d", &start, &ende);
	
	for (i=0; i<MAXNODES; i++) startused[i] = 0;
	
	while ((start!=0) && (ende!=0))
	{
		if (startused[start]==0)
		{
			for (c='a'; c<='z'; c++) search(c);
			startused[start] = 1;
		}
		sols = 0;
		for (c='a'; c<='z'; c++)
		{
			if (reach[start][ende][c]==1)
			{
				sols++;
				printf("%c", c);
			}
		}
		if (sols == 0) printf("-");
		printf("\n");
		fscanf(infile, "%d %d", &start, &ende);
	}
	printf("\n");
}

int main()
{
	infile = fopen("fiber.in", "r");
	while (read_case()) solve_case();
	fclose(infile);
	return 0;
}