#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000000
#define Inf64 1000000000000000001LL

int main(){
	
	int n,m;
	cin>>n>>m;
	
	vector<int> u(m),v(m);
	int x = 0,y = 0;
	rep(i,m){
		cin>>u[i]>>v[i];
		u[i]--,v[i]--;
	}
	if(m%2==1){
		cout<<"-1"<<endl;
		return 0;
	}
	int R = 0,B = 0;
	string ans(m,'A');
	dsu D0(n),D1(n);
	rep(i,m){
		if(u[i]==0 && v[i]==n-1){
			cout<<-1<<endl;
			return 0;
		}
		if(u[i]!=0){
			D0.merge(u[i],v[i]);
		}
		if(v[i]!=n-1){
			D1.merge(u[i],v[i]);
		}
	}
	rep(i,m){
		if(u[i]==0){
			if(D0.same(n-1,v[i])){
				ans[i] = 'R';
				R++;
			}
		}
		if(v[i]==n-1){
			if(D1.same(u[i],0)){
				ans[i] = 'B';
				B++;
			}
		}
	}
	
	rep(i,m){
		if(ans[i]=='A'){
			if(R<B){
				ans[i] = 'R';
				R++;
			}
			else{
				ans[i]='B';
				B++;
			}
		}
	}
	cout<<ans<<endl;
	return 0;
}