#include <bits/stdc++.h>
using namespace std;
using ll = long long;

int main(){
    int n, k, p;
    char c;
    cin >> n >> k;
    vector<vector<pair<int,char>>> tb(3);
    for(int i = 0; i < k; i++){
        cin >> p >> c;
        p--;
        tb[p % 3].emplace_back(p, c);
    }
    string ans(n, '?');
    for(int i = 0; i < tb[0].size(); i++){
        tie(p, c) = tb[0][i];
        if(ans[p] != '?'){
            cout << -1 << '\n';
            continue;
        }
        ans[p] = c;
    }
    for(int i = 0; i < tb[1].size(); i++){
        tie(p, c) = tb[1][i];
        if(ans[p - 1] == c)continue;
        if(ans[p] == c)continue;
        if(ans[p - 1] == '?')ans[p - 1] = c;
        else if(ans[p] == '?')ans[p] = c;
        else{
            cout << -1 << '\n';
            return 0;
        }
    }
    for(int i = 0; i < n; i += 3){
        set<char> S{'R','G','B'};
        for(int j = 0; j < 2; j++){
            if(ans[i + j] != '?')S.erase(ans[i + j]);
        }
        int j = i + 2;
        while(!S.empty()){
            if(j < ans.size())ans[j--] = *S.rbegin();
            else j--;
            S.erase(*S.rbegin());
        }
    }
    cout << ans << endl;
}