/**
*    author:  shu8Cream
*    created: 19.08.2022 21:25:57
**/

#include <bits/stdc++.h>
using namespace std;
#define overload3(a,b,c,d,...) d
#define rep1(i,n) for (int i=0; i<(n); i++)
#define rep2(i,a,n) for (int i=(a); i<(n); i++)
#define rep(...) overload3(__VA_ARGS__, rep2, rep1)(__VA_ARGS__)
#define rrep1(i,n) for (int i=(n-1); i>=0; i--)
#define rrep2(i,a,n) for (int i=(n-1); i>=(a); i--)
#define rrep(...) overload3(__VA_ARGS__, rrep2, rrep1)(__VA_ARGS__)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) int((x).size())
#define pcnt __builtin_popcountll
using ll = long long;
using P = pair<ll,ll>;
template<typename T> using vc = vector<T>;
template<typename T> using vv = vc<vc<T>>;
using vi = vc<ll>;
using vvi = vv<ll>;
const int INF = 1e9;
const ll LINF = 8e18;
template<typename T>istream& operator>>(istream&i,vc<T>&v){rep(j,sz(v))i>>v[j];return i;}
template<typename T>string join(const T&v,const string& d=""){stringstream s;rep(i,sz(v))(i?s<<d:s)<<v[i];return s.str();}
template<typename T>ostream& operator<<(ostream&o,const vc<T>&v){if(sz(v))o<<join(v," ");return o;}
template<typename T1,typename T2>istream& operator>>(istream&i,pair<T1,T2>&v){return i>>v.first>>v.second;}
template<typename T1,typename T2>ostream& operator<<(ostream&o,const pair<T1,T2>&v){return o<<v.first<<","<<v.second;}
template<class T> inline bool chmax(T& a, T b) {if(a<b) { a=b;return true; } return false;}
template<class T> inline bool chmin(T& a, T b) {if(a>b) { a=b;return true; } return false;}

template <class T> string to_string(T s);
template <class S, class T> string to_string(pair<S, T> p);
string to_string(char c) { return string(1, c); }
string to_string(string s) { return s; }
string to_string(const char s[]) { return string(s); }

template <class T>
string to_string(T v) {
    if (v.empty()) return "{}";
    string ret = "{";
    for (auto x : v) ret += to_string(x) + ",";
    ret.back() = '}';
    return ret;
}
template <class S, class T>
string to_string(pair<S, T> p) {
    return "{" + to_string(p.first) + ":" + to_string(p.second) + "}";
}

void debug_out() { cout << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
    cout << to_string(H) << " ";
    debug_out(T...);
}

#ifdef _DEBUG
#define debug(...) debug_out(__VA_ARGS__)
#else
#define debug(...)
#endif

string rgb = "RGB";

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(15);
    ll n,k; cin >> n >> k;
    vi color(n+1);
    rep(i,k){
        int a; char c; cin >> a >> c;
        rep(j,3)if(rgb[j]==c) color[a] = j;
    }
    string ans = string(n,'R');
    vc<int> cnt(3);
    rep(a,1,n+1){
        int c = color[a];
        if(a%3==1){
            ans[a-1] = rgb[c];
            cnt[c]++;
        }else if(a%3==2){
            if(ans[a-2]!=rgb[c]){
                ans[a-1] = rgb[c];
                cnt[c]++;
            }else{
                int id = (c+1)%3;
                ans[a-1] = rgb[id];
                cnt[id]++;
            }
        }else{
            int p = 0, q = 0;
            rep(j,3){
                if(ans[a-1]==rgb[j]) p = j;
                if(ans[a-2]==rgb[j]) q = j;
            }
            if(p==q){
                ans[a-1] = rgb[(p+1)%3];
                cnt[(p+1)%3]++;
            }else{
                ans[a-1] = rgb[3-p-q];
                cnt[3-p-q]++;
            }
        }
    }
    bool f = false;
    rep(i,n)rep(j,3)if(ans[i]==j){
        int nx = (j+1)%3;
        int nnx = (nx+1)%3;
        if(cnt[j]<cnt[nx] || cnt[j]<cnt[nnx]) f = true;
    }
    if(f) ans="-1";
    cout << ans << endl;
}