#include <iostream>
#include <vector>
#include <cmath>
#include <map>
#include <set>
#include <iomanip>
#include <queue>
#include <algorithm>
#include <numeric>
#include <deque>
#include <complex>
#include <cassert>

using namespace std;
using ll = long long;

int main(){

    int N, K, now=0, M, A, R=0, G=0, B=0, x;
    char c;
    cin >> N >> K;
    vector<pair<int, char>> ac(K);
    for (int i=0; i<K; i++){
        cin >> A >> c; A--;
        ac[i] = {A, c};
    }
    sort(ac.begin(), ac.end());

    vector<char> ans(N, '.');
    queue<int> que;

    for (int i=0; i<K; i++){
        tie(A, c) = ac[i];
        while(now <= A){
            que.push(now);
            now++;
        }
        if (c == 'R'){
            M = max({G-R, B-R, (A-G-B-2*R+max(G, B)+2)/2});
            if (M < 0) M = 0;
            if (M > que.size()){
                cout << -1 << endl;
                return 0;
            }
            R += M;
            for (int j=0; j<M; j++){
                x = que.front();
                ans[x] = 'R';
                que.pop();
            }
        }
        if (c == 'G'){
            M = max({R-G, B-G, (A-R-B-2*G+max(R, B)+2)/2});
            if (M < 0) M = 0;
            if (M > que.size()){
                cout << -1 << endl;
                return 0;
            }
            G += M;
            for (int j=0; j<M; j++){
                x = que.front();
                ans[x] = 'G';
                que.pop();
            }
        }
        if (c == 'B'){
            M = max({G-B, R-B, (A-G-R-2*B+max(G, R)+2)/2});
            if (M < 0) M = 0;
            if (M > que.size()){
                cout << -1 << endl;
                return 0;
            }
            B += M;
            for (int j=0; j<M; j++){
                x = que.front();
                ans[x] = 'B';
                que.pop();
            }
        }
    }

    for (int i=0; i<N; i++){
        cout << (ans[i] == '.' ? ac[K-1].second : ans[i]);
    }
    cout << endl;

    return 0;
}