#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 Inf 1000000001


unsigned long xor128() {
	static unsigned long x=123456789, y=362436069, z=521288629, w=88675123;
	unsigned long t=(x^(x<<11));
	x=y; y=z; z=w;
	return (w=(w^(w>>19))^(t^(t>>8)));
}

int main(){
	
	int N,M;
	cin>>N>>M;
	
	vector<int> A(M),B(M);
	vector<vector<int>> E(N);
	rep(i,M){
		cin>>A[i]>>B[i];
		A[i]--;B[i]--;
		E[A[i]].push_back(B[i]);
	}
	
	rep(i,N){
		vector<bool> visited(N,false);
		visited[i] = true;
		queue<int> Q;
		Q.push(i);
		while(Q.size()>0){
			int u = Q.front();
			Q.pop();
			
			rep(j,E[u].size()){
				int v = E[u][j];
				if(visited[v])continue;
				visited[v] = true;
				Q.push(v);
			}
		}
		
		rep(j,N){
			if(!visited[j]){
				cout<<-1<<endl;
				return 0;
			}
		}
	}
	
	vector<int> dis(N,Inf);

	stack<int> Q;
	int X = xor128()%N;
	Q.push(X);
	dis[X] = 0;
	
	while(Q.size()>0){
		int u = Q.top();
		Q.pop();
		rep(i,E[u].size()){
			int v = E[u][i];
			if(dis[v]==Inf)continue;
			dis[v] = dis[u] + 1;
			Q.push(v);
		}
	}
	string t = "RBBR";
	string ans = "";
	rep(i,M){
		ans += t[dis[A[i]]%4];
	}
	cout<<ans<<endl;
	
    return 0;
}