#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using mint = atcoder::static_modint<998244353>;
// using mint = atcoder::static_modint<1000000007>;
using ld = long double;
using ll = long long;
#define mp(a,b) make_pair(a,b)
#define rep(i,s,n) for(int i=s; i<(int)n; i++)
using vl = vector<ll>;
using vvl = vector<vl>;
using vvvl = vector<vvl>;
using vvvvl = vector<vvvl>;
const vector<int> dx{1,0,-1,0},dy{0,1,0,-1};

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n,m;cin >> n >> m;
    vvl G(n);
    vl u(m),v(m);
    rep(i,0,m){
        cin >> u[i] >> v[i];
        u[i]--,v[i]--;
        G[u[i]].push_back(v[i]);
        G[v[i]].push_back(u[i]);
    }
    if(m&1){
        cout << -1;
        return 0;
    }
    rep(i,0,m){
        if(u[i]==0 && v[i]==n-1){
            cout << -1;
            return 0;
        }
    }
    queue<int> Q;
    vector<bool> used(n);
    Q.push(0);
    used[0]=true;
    while(Q.size()){
        auto x=Q.front();Q.pop();

        for(auto y:G[x])if(!used[y] && y!=n-1){
            used[y]=true;
            Q.push(y);
        }
    }

    string ans=string(m,'.');
    int cntr=0;
    rep(i,0,m)if(used[u[i]] && v[i]==n-1)ans[i]='R',cntr++;
    rep(i,0,n)used[i]=false;

    Q.push(n-1);
    used[n-1]=true;
    while(Q.size()){
        auto x=Q.front();Q.pop();

        for(auto y:G[x])if(!used[y] && y!=0){
            used[y]=true;
            Q.push(y);
        }
    }

    rep(i,0,m)if(u[i]==0 && used[v[i]])ans[i]='B';
    
    rep(i,0,m)if(ans[i]=='.'){
        if(cntr*2<m)ans[i]='R',cntr++;
        else ans[i]='B';
    }
    cout << ans;
}