#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;
typedef vector<bool> vb;
typedef vector<vb> vvb;
typedef vector<vvb> vvvb;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
#define rep(i,l,r) for(int i=(l);i<(r);i++)
#define all(a) begin(a),end(a)
#define sz(a) (int)(a).size()
#define F first
#define S second
template<typename T>ostream&operator<<(ostream&os,vector<T>v){rep(i,0,sz(v))os<<v[i]<<(i+1!=sz(v)?" ":"");return os;}
template<typename T>istream&operator>>(istream&is,vector<T>&v){for(T&in:v)is>>in;return is;}
template<class T>bool chmax(T&a,T b){if(a<b){a=b;return 1;}return 0;}
template<class T>bool chmin(T&a,T b){if(b<a){a=b;return 1;}return 0;}
#include<atcoder/dsu>
using namespace atcoder;
int main(){
	int N,M;cin>>N>>M;
	if(M%2){cout<<-1<<endl;return 0;}
	vector<pii>A(M);
	rep(i,0,M)cin>>A[i].F>>A[i].S;
	rep(i,0,M)A[i].F--,A[i].S--;
	if(count(all(A),pii(0,N-1))){cout<<-1<<endl;return 0;}
	vvl E(N);
	rep(i,0,M)E[A[i].F].emplace_back(A[i].S),E[A[i].S].emplace_back(A[i].F);
	
	string ans=string(M,'-');
	int R=M/2,B=M/2;
	if(sz(E[0])<=M/2&&sz(E[N-1])<=M/2){
		rep(i,0,M){
			if(A[i].F==0)ans[i]='R',R--;
			if(A[i].S==N-1)ans[i]='B',B--;
		}
		rep(i,0,M)if(ans[i]=='-'){
			if(R)ans[i]='R',R--;
			else ans[i]='B';
		}
	}
	else if(sz(E[0])>M/2){
		rep(i,0,M)if(A[i].S==N-1)ans[i]='B',B--;
		dsu D(N);
		rep(i,0,M)if(A[i].F)D.merge(A[i].F,A[i].S);
		assert(D.size(N-1)<=1+M-sz(E[0]));
		rep(i,0,M)if(A[i].S!=N-1){
			if(B){
				if(A[i].F==0&&!D.same(A[i].S,N-1))ans[i]='B',B--;
				else if(A[i].F)ans[i]='B',B--;
				else ans[i]='R',R--;
			}
			else ans[i]='R';
		}
	}
	else{
		rep(i,0,M)if(A[i].F==0)ans[i]='B',B--;
		dsu D(N);
		rep(i,0,M)if(A[i].S!=N-1)D.merge(A[i].F,A[i].S);
		assert(D.size(0)<=1+M-sz(E[N-1]));
		rep(i,0,M)if(A[i].F){
			if(B){
				if(A[i].S==N-1&&!D.same(A[i].F,0))ans[i]='B',B--;
				else if(A[i].S!=N-1)ans[i]='B',B--;
				else ans[i]='R',R--;
			}
			else ans[i]='R';
		}
	}
	cout<<ans<<endl;
}