#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;
vector<pair<int,int>>G[200009];
int color[200009];
int main(){
    cin>>N>>M;
    vector<tuple<int,int,int>>vu(M);
    if(M%2==1){
        cout<<"-1"<<endl;
        return 0;
    }
    rep(i,M){
        cin>>get<0>(vu.at(i))>>get<1>(vu.at(i));
        get<2>(vu.at(i))=i+1;
        if(get<0>(vu.at(i))==1 && get<1>(vu.at(i))==N){
            cout<<-1<<endl;
            return 0;
        }
        if(get<0>(vu.at(i))==N && get<1>(vu.at(i))==1){
            cout<<-1<<endl;
            return 0;
        }
    }
    sort(vu.rbegin(),vu.rend());
    rep(i,M){
        G[get<0>(vu.at(i))].push_back(make_pair(get<1>(vu.at(i)),get<2>(vu.at(i))));
        G[get<1>(vu.at(i))].push_back(make_pair(get<0>(vu.at(i)),get<2>(vu.at(i))));
    }
    for(int i=1;i<=N;i++)
    for(int i=1;i<=M;i++) color[i]=0;
    queue<int>Q;
    Q.push(1);
    int cnt=0;
    while(!Q.empty()){
        int pos=Q.front();
        rep(i,G[pos].size()){
            if(G[pos][i].first!=N && color[G[pos][i].second]==0){
                color[G[pos][i].second]=1;
                Q.push(G[pos][i].first);
                cnt++;
            }
        }
        if(cnt==M/2){
            break;
        }
        Q.pop();
    }
    if(cnt==N/2){
        for(int i=1;i<=M;i++){
            if(color[i]==0) cout<<"B";
            else cout<<"R";
        }
        cout<<endl;
    }
    else{
        cout<<"-1"<<endl;
    }
}