#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i,s,n) for (int i = (s); i < (n); ++i)
#define rrep(i,n,g) for (int i = (n)-1; i >= (g); --i)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define len(x) (int)(x).size()
#define dup(x,y) (((x)+(y)-1)/(y))
#define pb push_back
#define eb emplace_back
#define Field(T) vector<vector<T>>
#define pq(T) priority_queue<T, vector<T>, greater<T>>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using P = pair<int,int>;
template<class T>bool chmax(T&a,T b){if(a<b){a=b;return 1;}return 0;}
template<class T>bool chmin(T&a,T b){if(b<a){a=b;return 1;}return 0;}

int main() {
  int n, m;
  cin >> n >> m;
  if (m % 2 == 1) {
    cout << -1 << endl;
    return 0;
  }
  vector<int> a(m), b(m);
  vector<int> d(n);
  rep(i,0,m) {
    cin >> a[i] >> b[i];
    --a[i], --b[i];
    ++d[a[i]], ++d[b[i]];
    if (a[i] == 0 && b[i] == n-1) {
      cout << -1 << endl;
      return 0;
    }
  }
  if (d[0] <= m/2 && d[n-1] <= m/2) {
    string ans(m, '.');
    int cnt = m/2;
    rep(i,0,m) {
      if (a[i] == 0) ans[i] = 'R', --cnt;
      if (b[i] == n-1) ans[i] = 'B';
    }
    rep(i,0,m) {
      if (ans[i] == '.') {
        if (cnt) ans[i] = 'R', --cnt;
        else ans[i] = 'B';
      }
    }
    cout << ans << endl;
    return 0;
  }
  if (d[0] > d[n-1]) {
    reverse(all(d));
    rep(i,0,m) {
      a[i] = n-a[i]-1, b[i] = n-b[i]-1;
      swap(a[i], b[i]);
    }
  }
  vector<vector<int>> G(n);
  rep(i,0,m) {
    if (b[i] == n-1) continue;
    G[a[i]].eb(b[i]);
    G[b[i]].eb(a[i]);
  }
  vector<int> seen(n, 0);
  queue<int> que;
  seen[0] = 1;
  que.emplace(0);
  while(!que.empty()) {
    int v = que.front();
    que.pop();
    for (int nv : G[v]) {
      if (seen[nv]) continue;
      seen[nv] = 1;
      que.emplace(nv);
    }
  }
  string ans(m, '.');
  int cnt = m/2;
  int cb = 0;
  rep(i,0,m) {
    if (a[i] == 0) ans[i] = 'R', --cnt;
    if (seen[a[i]] && b[i] == n-1) ans[i] = 'B', ++cb;
  }
  if (cb >= m/2) {
    cout << -1 << endl;
  }
  rep(i,0,m) {
    if (ans[i] == '.') {
      if (cnt) ans[i] = 'R', --cnt;
      else ans[i] = 'B';
    }
  }
  cout << ans << endl;
  return 0;
}