#include <bits/stdc++.h>
using namespace std;
 
#include <atcoder/all>
using namespace atcoder;
 
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
 
using ll = long long;
using mint = modint998244353;

int main() {
  int N, M;
  cin >> N >> M;
  bool no = false;
  vector<vector<pair<int, int>>> G(N);
  for(int i = 0; i < M; i++) {
    int u, v;
    cin >> u >> v;
    if(u == 1 && v == N) no = true;
    G[u - 1].push_back(make_pair(v - 1, i));
    G[v - 1].push_back(make_pair(u - 1, i));
  }
  if(M % 2 == 1 || no) {
    cout << -1 << "\n";
  }else if(G[0].size() > M / 2) {
    int r = 0, b = 0;
    string S(M, '?');
    unordered_set<int> st;
    for(const auto [v, i] : G[N - 1]) {
      st.insert(v);
      S[i] = 'R';
      r++;
    }
    for(const auto [v, i] : G[0]) {
      if(st.count(v)) {
        S[i] = 'B';
        b++;
      }
    }
    for(auto g : G) {
      for(const auto [v, i] : g) {
        if(S[i] != '?') continue;
        if(r < M / 2) S[i] = 'R', r++;
        else S[i] = 'B', b++;
      }
    }
    cout << S << endl;
  }else {
    int r = 0, b = 0;
    string S(M, '?');
    unordered_set<int> st;
    for(const auto [v, i] : G[0]) {
      st.insert(v);
      S[i] = 'R';
      r++;
    }
    for(const auto [v, i] : G[N - 1]) {
      if(st.count(v)) {
        S[i] = 'B';
        b++;
      }
    }
    for(auto g : G) {
      for(const auto [v, i] : g) {
        if(S[i] != '?') continue;
        if(r < M / 2) S[i] = 'R', r++;
        else S[i] = 'B', b++;
      }
    }
    cout << S << endl;
  }
}