#include <iostream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstdlib>
#include <cassert>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <bitset>
#include <string>
#include <algorithm>
#include <utility>
#include <complex>
#define rep(x, s, t) for(llint (x) = (s); (x) <= (t); (x)++)
#define reps(x, s) for(llint (x) = 0; (x) < (llint)(s).size(); (x)++)
#define chmin(x, y) (x) = min((x), (y))
#define chmax(x, y) (x) = max((x), (y))
#define sz(x) ((ll)(x).size())
#define ceil(x, y) (((x)+(y)-1) / (y))
#define all(x) (x).begin(),(x).end()
#define outl(...) dump_func(__VA_ARGS__)
#define inf 1e18

using namespace std;

typedef long long llint;
typedef long long ll;
typedef pair<ll, ll> P;

struct edge{
	ll to, cost;
	edge(){}
	edge(ll a, ll b){ to = a, cost = b;}
};

const ll mod = 1000000007;
//const ll mod = 998244353;

struct mint{
	ll x = 0;
	mint(ll y = 0){x = y; if(x < 0 || x >= mod) x = (x%mod+mod)%mod;}
	mint(const mint &ope) {x = ope.x;}
	
	mint operator-(){return mint(-x);}
	mint operator+(const mint &ope){return mint(x) += ope;}
	mint operator-(const mint &ope){return mint(x) -= ope;}
	mint operator*(const mint &ope){return mint(x) *= ope;}
	mint operator/(const mint &ope){return mint(x) /= ope;}
	mint& operator+=(const mint &ope){
		x += ope.x;
		if(x >= mod) x -= mod;
		return *this;
	}
	mint& operator-=(const mint &ope){
		x += mod - ope.x;
		if(x >= mod) x -= mod;
		return *this;
	}
	mint& operator*=(const mint &ope){
		x *= ope.x, x %= mod;
		return *this;
	}
	mint& operator/=(const mint &ope){
		ll n = mod-2; mint mul = ope;
		while(n){
			if(n & 1) *this *= mul;
			mul *= mul;
			n >>= 1;
		}
		return *this;
	}
	mint inverse(){return mint(1) / *this;}
	bool operator ==(const mint &ope){return x == ope.x;}
	bool operator !=(const mint &ope){return x != ope.x;}
};
istream& operator >>(istream &is, mint &ope){
	ll t; is >> t, ope.x = t;
	return is;
}
ostream& operator <<(ostream &os, mint &ope){return os << ope.x;}
ostream& operator <<(ostream &os, const mint &ope){return os << ope.x;}

bool exceed(ll x, ll y, ll m){return x >= m / y + 1;}
void mark(){ cout << "*" << endl; }
void yes(){ cout << "Yes" << endl; }
void no(){ cout << "No" << endl; }
ll gcd(ll a, ll b){if(b == 0) return a; return gcd(b, a%b);}
ll modpow(ll a, ll n){
	if(n == 0) return 1;
	if(n % 2) return ((a%mod) * (modpow(a, n-1)%mod)) % mod;
	else return modpow((a*a)%mod, n/2) % mod;
}


template<typename T>
ostream& operator << (ostream& os, vector<T>& vec) {
	for(int i = 0; i < vec.size(); i++) {
		os << vec[i] << (i + 1 == vec.size() ? "" : " ");
	}
	return os;
}
template<typename T>
ostream& operator << (ostream& os, deque<T>& deq) {
	for(int i = 0; i < deq.size(); i++) {
		os << deq[i] << (i + 1 == deq.size() ? "" : " ");
	}
	return os;
}
template<typename T, typename U>
ostream& operator << (ostream& os, pair<T, U>& pair_var) {
	os << "(" << pair_var.first << ", " << pair_var.second << ")";
	return os;
}
template<typename T, typename U>
ostream& operator << (ostream& os, const pair<T, U>& pair_var) {
	os << "(" << pair_var.first << ", " << pair_var.second << ")";
	return os;
}
template<typename T, typename U>
ostream& operator << (ostream& os, map<T, U>& map_var) {
	for(typename map<T, U>::iterator itr = map_var.begin(); itr != map_var.end(); itr++) {
		os << "(" << itr->first << ", " << itr->second << ")";
		itr++;
		if(itr != map_var.end()) os << ",";
		itr--;
	}
	return os;
}
template<typename T>
ostream& operator << (ostream& os, set<T>& set_var) {
	for(typename set<T>::iterator itr = set_var.begin(); itr != set_var.end(); itr++) {
		os << *itr;
		++itr;
		if(itr != set_var.end()) os << " ";
		itr--;
	}
	return os;
}
template<typename T>
void outa(T a[], ll s, ll t)
{
	for(ll i = s; i <= t; i++){ cout << a[i]; if(i < t) cout << " ";}
	cout << endl;
}
void dump_func() {cout << endl;}
template <class Head, class... Tail>
void dump_func(Head &&head, Tail &&... tail) {
	cout << head;
	if(sizeof...(Tail) > 0) cout << " ";
	dump_func(std::move(tail)...);
}


ll n, m;
vector<ll> G[1005], g[2005];
ll dist[2005][2005], pre[2005][2005];
ll eid[1005][1005];
ll ans[2005];
bool used[2005];

void bfs(vector<llint> G[], int S, llint dist[], llint pre[])
{
	queue<int> Q;
	Q.push(S);
	
	for(int i = 1; i <= 2*n; i++) dist[i] = inf;
	dist[S] = 0;
	
	while(Q.size()){
		int v = Q.front();
		Q.pop();
		for(int i = 0; i < G[v].size(); i++){
			if(dist[G[v][i]] < inf/2) continue;
			Q.push(G[v][i]);
			dist[G[v][i]] = dist[v] + 1;
			pre[G[v][i]] = v;
		}
	}
}

int main(void)
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	cin >> n >> m;
	ll u, v;
	rep(i, 1, m){
		cin >> u >> v;
		eid[u][v] = i;
		G[u].push_back(v);
		g[u].push_back(n+v);
		g[n+u].push_back(v);
	}
	
	rep(i, 1, 2*n){
		bfs(g, i, dist[i], pre[i]);
		rep(j, 1, 2*n){
			if(dist[i][j] > inf/2){
				outl(-1);
				return 0;
			}
		}
	}
	
	vector<ll> vec;
	rep(i, 1, n){
		ll v = n+i; vector<ll> tmp;
		while(pre[i][v]){
			v = pre[i][v];
			if(v > n) tmp.push_back(v-n);
			else tmp.push_back(v);
		}
		map<ll, ll> mp;
		for(auto x : tmp) mp[x]++;
		
		bool flag = true;
		for(auto p : mp) if(p.second >= 2) flag = false;
		if(flag && (sz(vec) == 0 || sz(vec) > sz(tmp))) vec = tmp;
	}
	reverse(all(vec));
	
	bool flag = false;
	for(auto v : vec) if(sz(G[v]) > 1) flag = true;
	if(!flag){
		outl(-1);
		return 0;
	}
	
	
	reps(i, vec) used[eid[vec[i]][vec[(i+1)%sz(vec)]]] = true;
	rep(i, 1, n){
		g[i].clear();
		for(auto u : G[i]){
			if(used[eid[i][u]]) continue;
			g[i].push_back(u);
		}
	}
	
	ll s = vec.front(), t = vec.front();
	for(auto v : vec){
		bfs(g, v, dist[v], pre[v]);
		for(auto u : vec){
			if(u != v && dist[v][u] < inf/2){
				s = v, t = u;
				ll x = u;
				while(pre[v][x]){
					ll nx = pre[v][x];
					ans[eid[nx][x]] = 1;
					x = nx;
				}
				goto end;
			}
		}
	}
	end:;
	
	ll p;
	reps(i, vec){
		if(vec[i] == t){
			p = i;
			break;
		}
	}
	
	ll cnt = 1;
	while(1){
		ll v = vec[p%sz(vec)], nv = vec[(p+1)%sz(vec)];
		ans[eid[v][nv]] = cnt % 2, cnt++;
		if(nv == s) break;
		p++;
	}
	
	rep(i, 1, m){
		if(ans[i]) cout << "R";
		else cout << "G";
	}
	cout << endl;
	
	return 0;
}