#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n, k; cin >> n >> k;
	map<int, char>mp;
	rep(i, k) {
		int a; cin >> a;
		char c; cin >> c;
		mp[a - 1] = c;
	}
	int cr = 0, cg = 0, cb = 0;
	string ans;
	rep(i, n) {
		if ((cr == cg) && (cr == cb)) {
			if (mp.count(i)) {
				char c = mp[i];
				if ('R' == c)cr++;
				else if ('G' == c)cg++;
				else cb++;
				ans += c;
			}
			else{
				cb++;
				ans += 'B';
			}
		}
		else {
			if ((cr <= cg) && (cr <= cb)) {
				cr++;
				ans += 'R';
			}
			else if ((cg <= cr) && (cg <= cb)) {
				cg++;
				ans += 'G';
			}
			else {
				cb++;
				ans += 'B';
			}
		}
		//cout << i + 1 << endl;
		//cout << ans << endl;
		//cout << cr << cb << cg << endl;
	}
	cout << -1 << endl;
	//cout << ans << endl;
	return 0;
}