#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
#define rep(i,n) for(int i=0; i<(int)(n); i++)
#define rrep(i, n) for(int i=((int)(n)-1); i>=0; --i)
#define all(x) (x).begin(),(x).end()
#define MOD 1000000007
#define MOD2 998244353
#define INF 1000000007
#define LINF 1000000000000000007LL
#define PI 3.14159265359
#define P pair<ll,ll>
template <typename T>
inline bool chmax(T &a, const T &b){
    if(a<b) {a=b; return true;}
    else return false;
}
template <typename T>
inline bool chmin(T &a, const T &b){
    if(a>b) {a=b; return true;}
    else return false;
}
struct Edge{
    int to; ll cost; int rev;
    Edge(int to, ll cost, int rev) : to(to), cost(cost), rev(rev) {}
};
typedef vector<Edge> Edges;
typedef vector<Edges> Graph;
void add_edge(Graph &g,int from,int to,ll cost,bool revFlag,ll revCost){
    g[from].push_back(Edge(to,cost,g[to].size()));
    if(revFlag) g[to].push_back(Edge(from,revCost,g[from].size()-1));
}

string st = "RGB";

void solve(){
    int n,k;
    cin>>n>>k;
    map<int,int> mp;
    rep(i,k){
        int a; char c;
        cin>>a>>c;
        int col;
        if(c=='R') col=1;
        if(c=='G') col=2;
        if(c=='B') col=3;
        mp[--a]=col;
    }

    string ans="";
    vector<int> col(3,0);
    int m=1;
    bool exist=1;
    rep(i,n){
        if(mp[i]==0){
            m++;
            continue;
        }
        int c=mp[i]-1;
        int minc;
        rep(j,m){
            int cnt=INF;
            rep(x,3){
                if(chmin(cnt,col[x])) minc=x;
            }
            if((col[c]<col[0] || col[c]<col[1] || col[c]<col[2]) || (col[0]==col[1] && col[1]==col[2])){
                ans+=st[c];
                col[c]++;
            }else{
                ans+=st[minc];
                col[minc]++;
            }
        }
        if(col[c]<col[0] || col[c]<col[1] || col[c]<col[2]) exist=0;
        m=1;
    }
    rep(j,m-1) ans += 'R';
    if(!exist) cout<<-1<<endl;
    else cout<<ans<<endl;
}   

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout<<setprecision(10)<<fixed;
    solve();
    return 0;
}