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

#define rep(i,n) for(int i = 0; i < (int)n; i++)
#define FOR(n) for(int i = 0; i < (int)n; i++)
#define repi(i,a,b) for(int i = (int)a; i < (int)b; i++)
#define pb push_back
#define m0(x) memset(x,0,sizeof(x))
#define fill(x,y) memset(x,y,sizeof(x))
#define bg begin()
#define ed end()
#define all(x) x.bg,x.ed
//#define mp make_pair
#define vi vector<int>
#define vvi vector<vi>
#define vll vector<ll>
#define vvll vector<vll>
#define vs vector<string>
#define vvs vector<vs>
#define vc vector<char>
#define vvc vector<vc>
#define pii pair<int,int>
#define pllll pair<ll,ll>
#define vpii vector<pair<int,int>>
#define vpllll vector<pair<ll,ll>>
#define vpis vector<pair<int,string>>
#define vplls vector<pair<ll, string>>
#define vpsi vector<pair<string, int>>
#define vpsll vector<pair<string, ll>>

template<typename T>
void chmax(T &a, const T &b) {a = (a > b? a : b);}
template<typename T>
void chmin(T &a, const T &b) {a = (a < b? a : b);}

using ll = long long;
using ld = long double;
using ull = unsigned long long;

const ll INF = numeric_limits<long long>::max() / 2;
const ld pi = 3.1415926535897932384626433832795028;
const ll mod = 1e9 + 7;
int dx[] = {-1, 0, 1, 0, -1, -1, 1, 1};
int dy[] = {0, -1, 0, 1, -1, 1, -1, 1};

#define int long long

void solve() {
    int n, k;
    cin >> n >> k;
    vi a(k);
    vector<char> c(k);

    FOR(k) cin >> a[i] >> c[i], a[i]--;

    vector<int> order(k);
    iota(all(order), 0);
    sort(all(order), [&](int i, int j) {
        return a[i] < a[j];
    });

    int r = 0, g = 0, b = 0;
    int pre = -1;
    string ans = "";

    for(int i : order) {
        repi(j, pre+1, a[i] + 1) {
            if(c[i] == 'R') {
                if(r <= g && r <= b) r++, ans.push_back('R');
                else if(g < b) g++, ans.push_back('G');
                else b++, ans.push_back('B');
            }else if(c[i] == 'G') {
                if(g <= r && g <= b) g++, ans.push_back('G');
                else if(r < b) r++, ans.push_back('R');
                else b++, ans.push_back('B');
            }else {
                if(b <= g && b <= r) b++, ans.push_back('B');
                else if(g < r) g++, ans.push_back('G');
                else r++, ans.push_back('R');
            }
        }

        if(c[i] == 'R') {
            if(r < g || r < b) {
                cout << -1 << endl;
                return;
            }
        }else if(c[i] == 'G') {
            if(g < r || g < b) {
                cout << -1 << endl;
                return;
            }
        }else {
            if(b < r || b < g) {
                cout << -1 << endl;
                return;
            }
        }
        pre = a[i];
    }

    repi(j, pre+1, n) ans.push_back('R');

    cout << ans << endl;
}

signed main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    solve();
    return 0;
}