#include<bits/stdc++.h>
// #include<atcoder/all>
// #include<boost/multiprecision/cpp_int.hpp>

using namespace std;
// using namespace atcoder;
// using bint = boost::multiprecision::cpp_int;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using vi = vector<ll>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using ve = vector<vector<int>>;
using vb = vector<bool>;
using vvb = vector<vb>;
#define rep(i,s,n) for(ll i = (ll)s;i < (ll)n;i++)
#define rrep(i,l,r) for(ll i = r-1;i >= l;i--)
#define ALL(x) (x).begin(),(x).end()
#define sz(c) ((ll)(c).size())
#define LB(A,x) (int)(lower_bound(A.begin(),A.end(),x)-A.begin())
#define UB(A,x) (int)(upper_bound(A.begin(),A.end(),x)-A.begin())
// #define MOD 1000000007
#define MOD 998244353
template<typename T>using min_priority_queue=priority_queue<T,vector<T>,greater<T>>;
template<typename T>ostream&operator<<(ostream&os,vector<T>&v){for(int i = 0;i < v.size();i++)os<<v[i]<<(i+1!=v.size()?" ":"");return os;}
template<typename T>istream&operator>>(istream&is,vector<T>&v){for(T&in:v)is>>in;return is;}
template<typename T1,typename T2>ostream&operator<<(ostream&os,pair<T1,T2>&p){os<<p.first<<" "<<p.second;return os;}
template<typename T1,typename T2>istream&operator>>(istream&is,pair<T1,T2>&p){is>>p.first>>p.second;return is;}
template<typename T> inline bool chmax(T &a,T b){if(a < b){a = b;return true;}return false;}
template<typename T> inline bool chmin(T &a,T b){if(a > b){a = b;return true;}return false;}
ld dist(ld x1,ld y1,ld x2, ld y2){return sqrtl((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));}


int main(){
  
  ios_base::sync_with_stdio(0), cin.tie(0);
  int n,m;cin >> n >> m;
  vector<pii> v(n,{-1,-1});
  bool is = !(m & 1);
  int c0 = 0,c1 = 0;
  rep(i,0,m){
    int a,b;cin >> a >> b;
    a--;b--;
    if(a == 0)v[b].first = i,c0++;
    if(b == n-1)v[a].second = i,c1++;
    if(a == 0 && b == n-1)is = false;
  }
  if(!is){
    cout << -1 << endl;
    return 0;
  }
  string res = string(m,'G');
  if(c0 <= m/2 && c1 <= m/2){
    int cnt0 = m/2,cnt1 = m/2;
    rep(i,0,n){
      if(v[i].first != -1)res[v[i].first] = 'R',cnt0--;
      if(v[i].second != -1)res[v[i].second] = 'B',cnt1--;
    }
    rep(i,0,m){
      if(res[i] == 'G'){
        if(cnt0)res[i] = 'R',cnt0--;
        else res[i] = 'B',cnt1--;
      }
    }
  }else{
    if(c1 <= m/2){
      rep(i,0,n)swap(v[i].first,v[i].second);
    }
    c0 = m/2;
    rep(i,0,n){
      if(v[i].first != -1){
        res[v[i].first] = 'B';c0--;
        v[i].first = -1;
        if(v[i].second != -1){
          res[v[i].second] = 'R';c1--;
          v[i].second = -1;
        }
      }
    }
    rep(i,0,n){
      if(v[i].second != -1 && c0){
        res[v[i].second] = 'B';c0--;
        v[i].second = -1;
      }
    }
    rep(i,0,n){
      if(v[i].second != -1){
        res[v[i].second] = 'R';
      }
    }
    rep(i,0,m)if(res[i] == 'G')res[i] = 'R';
  }
  // assert(count(ALL(res),'R') == count(ALL(res),'B'));
  cout << res << endl;
  

  return 0;
}