#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];
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));
    }
    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;
    }
}