#ifdef NACHIA
#define _GLIBCXX_DEBUG
#else
#define NDEBUG
#endif
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

namespace nachia {

struct DsuFast{
private:
    std::vector<int> w;
public:
    DsuFast(int n = 0) : w(n, -1) {}
    int leader(int u){
        if(w[u] < 0) return u;
        return w[u] = leader(w[u]);
    }
    int operator[](int u){ return leader(u); }
    int merge(int u, int v){
        u = leader(u);
        v = leader(v);
        if(u == v) return u;
        if(-w[u] < -w[v]) std::swap(u, v);
        w[u] += w[v];
        w[v] = u;
        return u;
    }
    int size(int u){ return -w[leader(u)]; }
    bool same(int u, int v){ return leader(u) == leader(v); }
};

} // namespace nachia
using i64 = long long;
using u64 = unsigned long long;
#define rep(i,n) for(i64 i=0; i<i64(n); i++)
const i64 INF = 1001001001001001001;
template<typename A> void chmin(A& l, const A& r){ if(r < l) l = r; }
template<typename A> void chmax(A& l, const A& r){ if(l < r) l = r; }
using namespace std;

void testcase(){
    i64 N, M; cin >> N >> M;
    bool ok = true;
    vector<i64> U(M), V(M), D(N);
    rep(i,M){
        i64 u,v; cin >> u >> v; u--; v--;
        if(u == 0 && v == N-1) ok = false;
        U[i] = u; V[i] = v;
        D[u]++; D[v]++;
    }
    if(!ok || M%2 == 1){ cout << "-1\n"; return; }
    if(D[0] > M/2){
        for(auto& u : U) u = N-1-u;
        for(auto& u : V) u = N-1-u;
        swap(U, V);
    }
    auto dsu = nachia::DsuFast(N);
    string ans = string(M, 'R');
    i64 cnt = 0;
    auto check = [&](i64 i){
        if(cnt == M/2) return;
        cnt++; ans[i] = 'B';
        dsu.merge(U[i], V[i]);
    };
    rep(i,M) if(U[i] == 0) check(i);
    rep(i,M) if(ans[i] != 'B' && V[i] != N-1) check(i);
    rep(i,M) if(V[i] == N-1 && !dsu.same(0, U[i])) check(i);
    cout << ans << "\n";
}

int main(){
    ios::sync_with_stdio(false); cin.tie(nullptr);
    rep(t,1) testcase();
    return 0;
}