#include <stdio.h>
#define MAX 10100

int degree[MAX];
int visited[MAX];
int nasl[MAX][MAX];
int poz[MAX];
int maska[MAX*MAX];
int maska2[MAX*MAX];
int pocet;
int N, E;

int DFS(int v)
{
	if (visited[v])
		return 0;
	visited[v] = 1;
	//printf("DFS %d\n", v);

	int sum = 0;
	for (int i = 0; i < degree[v]; i++)
	{
		int ret = DFS(nasl[v][i]); 		
		if (ret)
		{
			maska[pocet] = nasl[v][i];
			maska2[pocet++] = v;
		}
		sum += ret;
	}
	return (sum + poz[v]) % 2;
}

int main()
{
	while (1)
	{
		scanf("%d %d\n", &N, &E);
		//printf("%d %d\n", N, E);
		if (N == 0)
			break;
		for (int i = 0; i < N; i++)
			degree[i] = 0;
		for (int i = 0; i < N; i++)
			visited[i] = 0;
		for (int i = 0; i < E; i++)
		{
			int a, b; scanf("%d %d\n", &a, &b); a--; b--;
			nasl[a][degree[a]++] = b;
			nasl[b][degree[b]++] = a;
		}
		for (int i = 0; i < N; i++)
		{
			char ch = getchar();
			if (ch == 'e')
				poz[i] = 0;
			else
				poz[i] = 1;
		}

		pocet = 0;
		
		int bad = 0;
		for (int j = 0; j < N; j++)
		{
			if (DFS(j))
				bad = 1;
		}
		if (bad)
		{
			printf("impossible\n");
		}
		else
		{
			printf("%d\n", pocet);
			for (int i = 0; i < pocet; i++)
			{
				printf("%d %d\n", maska[i]+1, maska2[i]+1);				
			}
		}
	}
	return 0;
}