#include<bits/stdc++.h>
#include<atcoder/all>
using namespace std;
using namespace atcoder;
typedef long long int ll;
typedef long double ld;
#define FOR(i,l,r) for(ll i=l;i<r;i++)
#define REP(i,n) FOR(i,0,n)
#define RFOR(i,l,r) for(ll i=r-1;i>=l;i--)
#define RREP(i,n) RFOR(i,0,n)
#define ALL(x) x.begin(),x.end()
#define PA pair<ll,ll>
#define F first
#define S second
#define BS(A,x) binary_search(ALL(A),x)
#define LB(A,x) (ll)(lower_bound(ALL(A),x)-A.begin())
#define UB(A,x) (ll)(upper_bound(ALL(A),x)-A.begin())
#define COU(A,x) (UB(A,x)-LB(A,x))
#define sz(c) ((ll)(c).size())
/*
#include<boost/multiprecision/cpp_int.hpp>
namespace mp=boost::multiprecision;
using Bint=mp::cpp_int;
*/
template<typename T>using min_priority_queue=priority_queue<T,vector<T>,greater<T>>;
template<typename T1,typename T2>ostream&operator<<(ostream&os,pair<T1,T2>&p){os<<p.F<<" "<<p.S;return os;}
template<typename T1,typename T2>istream&operator>>(istream&is,pair<T1,T2>&p){is>>p.F>>p.S;return is;}
template<typename T>ostream&operator<<(ostream&os,vector<T>&v){REP(i,sz(v))os<<v[i]<<(i+1!=sz(v)?" ":"");return os;}
template<typename T>istream&operator>>(istream&is,vector<T>&v){for(T&in:v)is>>in;return is;}
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;}
ld dist(ld x1,ld y1,ld x2,ld y2){return sqrtl((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));}
using mint=modint1000000007;
//using mint=modint998244353;
const ll mod=1000000007;
//const ll mod=998244353;
//using mint=modint;
ostream&operator<<(ostream&os,mint&a){os<<a.val();return os;}

vector<ll>fast_mod_convolution(vector<ll>&a,vector<ll>&b,ll mod){
  const ll m1=167772161,m2=469762049,m3=1224736769;
  const ll m1_inv_m2=inv_mod(m1,m2);
  const ll m12_inv_m3=inv_mod(m1*m2,m3);
  const ll m12_mod=m1*m2%mod;
  auto x=convolution<m1>(a,b);
  auto y=convolution<m2>(a,b);
  auto z=convolution<m3>(a,b);
  vector<ll>ret(sz(a)+1);
  REP(i,sz(a)+1){
    ll v1=(y[i]-x[i])*m1_inv_m2%m2;if(v1<0)v1+=m2;
    ll v2=(z[i]-(x[i]+m1*v1)%m3)*m12_inv_m3%m3;if(v2<0)v2+=m3;
	ret[i]=(x[i]+m1*v1+m12_mod*v2)%mod;
  }
  return ret;
}

int main(){
  ll N,K;cin>>N>>K;
  vector<PA>A(K);
  string T="RGB";
  REP(i,K){
    cin>>A[i].F;
    char c;cin>>c;
    A[i].S=find(ALL(T),c)-T.begin();
  }
  sort(ALL(A));
  string ans;
  ll t=0;
  vector<ll>B(3);
  REP(i,K){
    REP(j,A[i].F-t){
      if(max({B[0],B[1],B[2]})>B[A[i].S]||(B[0]==B[1]&&B[1]==B[2])){B[A[i].S]++;ans+=T[A[i].S];}
      else{
        ll m=min({B[0],B[1],B[2]});
        ll t=find(ALL(B),m)-B.begin();
        ans+=T[t];B[t]++;
      }
    }
    if(B[A[i].S]!=max({B[0],B[1],B[2]})){cout<<-1<<endl;return 0;}
    t=A[i].F;
  }
  cout<<ans<<endl;
  return 0;
}