#include <bits/stdc++.h>
using namespace std;
using ll=long long;
const ll ILL=2167167167167167167;
const int INF=2100000000;
#define rep(i,a,b) for (int i=(int)(a);i<(int)(b);i++)
#define all(p) p.begin(),p.end()
template<class T> using _pq = priority_queue<T, vector<T>, greater<T>>;
template<class T> int LB(vector<T> &v,T a){return lower_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> int UB(vector<T> &v,T a){return upper_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> bool chmin(T &a,T b){if(b<a){a=b;return 1;}else return 0;}
template<class T> bool chmax(T &a,T b){if(a<b){a=b;return 1;}else return 0;}
template<class T> void So(vector<T> &v) {sort(v.begin(),v.end());}
template<class T> void Sore(vector<T> &v) {sort(v.begin(),v.end(),[](T x,T y){return x>y;});}
bool yneos(bool a,bool upp=false){if(a){cout<<(upp?"YES\n":"Yes\n");}else{cout<<(upp?"NO\n":"No\n");}return a;}
template<class T> void vec_out(vector<T> &p,int ty=0){
    if(ty==2){cout<<'{';for(int i=0;i<(int)p.size();i++){if(i){cout<<",";}cout<<'"'<<p[i]<<'"';}cout<<"}\n";}
    else{if(ty==1){cout<<p.size()<<"\n";}for(int i=0;i<(int)(p.size());i++){if(i) cout<<" ";cout<<p[i];}cout<<"\n";}}
template<class T> T vec_min(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmin(ans,x);return ans;}
template<class T> T vec_max(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmax(ans,x);return ans;}
template<class T> T vec_sum(vector<T> &a){T ans=T(0);for(auto &x:a) ans+=x;return ans;}
int pop_count(long long a){int res=0;while(a){res+=(a&1),a>>=1;}return res;}
template<class T> T square(T a){return a * a;}



void solve();
// CITRUS CURIO CITY / FREDERIC
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int t = 1;
    // cin >> t;
    rep(i, 0, t) solve();
}

void solve(){
    int N, M;
    cin >> N >> M;
    vector<int> A(M), B(M);
    rep(i, 0, M) cin >> A[i] >> B[i], A[i]--, B[i]--;
    if (M & 1){
        cout << "-1\n";
        return;
    }
    rep(i, 0, M){
        if (A[i] == 0 && B[i] == N - 1){
            cout << "-1\n";
        }
    }
    vector<int> C(N);
    rep(i, 0, M) C[A[i]]++, C[B[i]]++;
    if (C[0] < C[N - 1]){
        reverse(all(C));
        rep(i, 0, M){
            A[i] = N - 1 - A[i];
            B[i] = N - 1 - B[i];
            swap(A[i], B[i]);
        }
    }
    vector<int> order = {N - 1};
    vector<int> seen(N);
    seen[N - 1] = 1;
    vector<vector<int>> G(N);
    rep(i, 0, M){
        G[A[i]].push_back(B[i]);
        G[B[i]].push_back(A[i]);
    }
    rep(rp, 0, order.size()){
        int a = order[rp];
        if (a == 0) continue;
        for (auto x : G[a]) if (seen[x] == 0){
            seen[x] = 1;
            order.push_back(x);
        }
    }
    int tmp = 0;
    string ans;
    rep(i, 0, M){
        if (A[i] == 0 && seen[B[i]] == 1){
            ans += "R";
            tmp++;
        }
        else if (B[i] == N - 1){
            ans += "B";
        }
        else ans += "?";
    }
    rep(i, 0, M){
        if (ans[i] == '?'){
            if (tmp != M / 2){
                tmp++;
                ans[i] = 'R';
            }
            else{
                ans[i] = 'B';
            }
        }
    }
    cout << ans << "\n";
}