#include<bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;

#define all(v) v.begin(),v.end()
using ll = long long;
using ull = unsigned long long;
using lll = __int128;
using vll=vector<ll>;
using vvll = vector<vector<ll>>;
using P = pair<ll,ll>;
using vp=vector<pair<ll, ll>>;
//using mint=modint1000000007;
//using mint=modint998244353;

const ll INF=1ll<<60;
ll mod10=1e9+7;
ll mod99=998244353;
const double PI = acos(-1);

#define rep(i,n) for (ll i=0;i<n;++i)
#define per(i,n) for(ll i=n-1;i>=0;--i)
#define rep2(i,a,n) for (ll i=a;i<n;++i)
#define per2(i,a,n) for (ll i=a;i>=n;--i)

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }

struct unionfind{
	ll N;
	vector<ll> par,Rank,Size;
	unionfind(ll n):N(n){
		par=vector<ll>(N),Rank=vector<ll>(N,1),Size=vector<ll>(N,1);
		for(int i=0;i<N;i++) par[i]=i;
	};
	ll find(ll x){
		if(par[x]==x) return x;
		else return par[x]=find(par[x]);
	}

	bool same(ll a,ll b){
		return find(a)==find(b);
	}

	void unite(ll a,ll b){
		if(same(a,b))return;
		a=find(a);
		b=find(b);
		ll s=Size[a]+Size[b];
		if(Rank[b]>Rank[a]) par[a]=b;
		else par[b]=a;
		if(Rank[a]==Rank[b]) Rank[a]++;
		Size[find(a)]=s;
	}

	ll size(ll a){
	   return Size[find(a)];
	}
};

bool solve(){
	ll N,M;cin>>N>>M;
	vector<vp> ab(N);
	rep(i,M){
		ll a,b;cin>>a>>b;
		a--;b--;
		ab[a].push_back({b,i});
		ab[b].push_back({a,i});
	}
	if(M%2==1){
		cout<<-1<<endl;
		return 0;
	}
	vll ans(M);
	queue<ll> q;
	q.push(0);
	vll dist(N,-1);
	ll cnt=0;
	while(q.size()){
		if(cnt==M/2) break;
		auto now=q.front();q.pop();
		for(auto [to,i]:ab[now]){
			if(to==N-1) continue;
			if(ans[i]==0) cnt++,ans[i]=1;
			if(dist[to]==-1){
				dist[to]=dist[now]+1;
				q.push(to);
			}
		}
	}

	unionfind uf0(N),uf1(N);
	rep(i,N){
		for(auto [to,j]:ab[i]){
			if(ans[j]==0) uf0.unite(i,to);
			else uf1.unite(i,to);
		}
	}

	if(uf0.same(0,N-1)||uf1.same(0,N-1)){
		cout << -1 << endl;
		return 0;
	}
	rep(i,M) cout<<(ans[i]==1?"B":"R");
	cout<<endl;
	return 0;
}



int main(){
   cin.tie(0);
   ios::sync_with_stdio(false);
   ll T=1;//cin>>T;
   rep(i,T) solve();
}