#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 1007

int N;
vector<vector<bool> >  G, U;
vector<bool> R, V;
bool par;

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

char buf[MAXN];

int main(){
	for(;;){
		scanf("%d", &N);
		if(N == 0)
			break;
		G.clear();
		G.resize(N, vector<bool>(N, false));

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

		scanf(" %s", buf);
		R.clear();
		R.resize(N);
		REP(i, N)
			R.at(i) = (buf[i] == 'o');

		U.clear();
		U.resize(N, vector<bool>(N, false));

		V.clear();
		V.resize(N, false);

		bool ok = true;
		REP(i, N) if(!V.at(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.at(i).at(j);
			printf("%d\n", res);
			REP(i, N) FOR(j, i + 1, N - 1)
				if(U.at(i).at(j))
					printf("%d %d\n", i + 1, j + 1);
		}
	}
	return 0;
}