#include <cstdio>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <algorithm>
#include <iostream>
#include <cstring>
#include <string>
#include <cassert>
#include <cmath>
#include <cstdlib>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef pair<int,int> PII;

#define pb push_back
#define fi first
#define se second
#define REP(i, n) for(int i = 0; i < (int)(n); ++i)
#define FOR(i, a, b) for(int i = (int)(a); i <= (int)(b); ++i)

#define MAXN 10007

int N;
bool G[MAXN][MAXN];
bool U[MAXN][MAXN];
bool R[MAXN];
bool V[MAXN];
bool par;

void DFS(int x){
	V[x] = true;
	if(R[x])
		par = !par;
	REP(i, N)
		if(G[x][i] && !V[i]){
			if(par)
				U[x][i] = U[i][x] = !U[x][i];
			DFS(i);
			if(par)
				U[x][i] = U[i][x] = !U[x][i];
		}
}

int main(){
	for(;;){
		scanf("%d", &N);
		if(N == 0)
			break;
		REP(i, N) REP(j, N)
			G[i][j] = false;

		int E;
		scanf("%d", &E);
		while(E--){
			int a, b;
			scanf("%d %d", &a, &b);
			--a, --b;
			G[a][b] = G[b][a] = true;
		}

		char buf[MAXN];
		scanf(" %s", buf);
		REP(i, N)
			R[i] = buf[i] == 'o';

		REP(i, N) REP(j, N)
			U[i][j] = false;

		REP(i, N)
			V[i] = false;

		bool ok = true;
		REP(i, N) if(!V[i]){
			par = false;
			DFS(i);
			if(par){
				ok = false;
				break;
			}
		}

		if(!ok)
			printf("impossible\n");
		else{
			int res = 0;
			REP(i, N) FOR(j, i + 1, N - 1)
				res += U[i][j];
			printf("%d\n", res);
			REP(i, N) FOR(j, i + 1, N - 1)
				if(U[i][j])
					printf("%d %d\n", i + 1, j + 1);
		}
	}
	return 0;
}