#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define ll long long
#define INF 9e18;
int N,M;
int u[200009],v[200009];
vector<pair<int,int>>G[200009];
bool already_point[200009];
bool already_line[200009];
int color[200009];
int main(){
    cin>>N>>M;
    if(M%2==1){
        cout<<-1<<endl;
        return 0;
    }
    for(int i=1;i<=M;i++){
        cin>>u[i]>>v[i];
        G[u[i]].push_back(make_pair(v[i],i));
        G[v[i]].push_back(make_pair(u[i],i));
        if((u[i]==1 && v[i]==N) || (u[i]==N && v[i]==1)){
            cout<<-1<<endl;
            return 0;
        }
    }
    for(int i=1;i<=M;i++) color[i]=-1;
    int blue=0,red=0;
    for(int i=1;i<=N;i++) already_point[i]=false;
    for(int i=1;i<=M;i++) already_line[i]=false;
    queue<int>Q;
    if(G[N].size()>M/2){
        Q.push(1);
        while(!Q.empty()){
            int pos=Q.front();
            if(pos==N){
                Q.pop();
                continue;
            }
            already_point[pos]=true;
            rep(i,G[pos].size()){
                if(already_point[G[pos][i].first]==false){
                    Q.push(G[pos][i].first);
                    if(G[pos][i].first==N) already_line[G[pos][i].second]=true;
                }
            }
            Q.pop();
        }
        for(int i=1;i<=M;i++){
            if(already_line[i]==true){
                color[i]=0;
                blue++;
            }
        }
    }
    else{
        rep(i,G[N].size()){
            color[G[N][i].second]=0;
            blue++;
        }
    }
    for(int i=1;i<=N;i++) already_point[i]=false;
    for(int i=1;i<=M;i++) already_line[i]=false;
    if(G[1].size()>M/2){
        Q.push(N);
        while(!Q.empty()){
            int pos=Q.front();
            if(pos==1){
                Q.pop();
                continue;
            }
            already_point[pos]=true;
            rep(i,G[pos].size()){
                if(already_point[G[pos][i].first]==false){
                    Q.push(G[pos][i].first);
                    if(G[pos][i].first==1) already_line[G[pos][i].second]=true;
                }
            }
            Q.pop();
        }
        for(int i=1;i<=M;i++){
            if(already_line[i]==true){
                color[i]=1;
                red++;
            }
        }
    }
    else{
        rep(i,G[1].size()){
            color[G[1][i].second]=1;
            red++;
        }
    }
    for(int i=1;i<=M;i++){
        if(color[i]==-1){
            if(blue<M/2){
                color[i]=0;
                blue++;
            }
            else color[i]=1;
        }
    }
    for(int i=1;i<=M;i++){
        if(color[i]==0) cout<<"B";
        else cout<<"R";
    }
    cout<<endl;
}