#include<bits/stdc++.h>
#include<atcoder/all>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
using namespace atcoder;
typedef long long ll;
typedef pair<int, int> P;

template <int m> ostream& operator<<(ostream& os, const static_modint<m>& a) {os << a.val(); return os;}
template <int m> ostream& operator<<(ostream& os, const dynamic_modint<m>& a) {os << a.val(); return os;}
template <int m> istream& operator>>(istream& is, static_modint<m>& a) {long long x; is >> x; a = x; return is;}
template <int m> istream& operator>>(istream& is, dynamic_modint<m>& a) {long long x; is >> x; a = x; return is;}
template<typename T> istream& operator>>(istream& is, vector<T>& v){int n = v.size(); assert(n > 0); rep(i, n) is >> v[i]; return is;}
template<typename U, typename T> ostream& operator<<(ostream& os, const pair<U, T>& p){os << p.first << ' ' << p.second; return os;}
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v){int n = v.size(); rep(i, n) os << v[i] << (i == n - 1 ? "\n" : " "); return os;}
template<typename T> ostream& operator<<(ostream& os, const vector<vector<T>>& v){int n = v.size(); rep(i, n) os << v[i] << (i == n - 1 ? "\n" : ""); return os;}
template<typename T> ostream& operator<<(ostream& os, const set<T>& se){for(T x : se) os << x << " "; os << "\n"; return os;}
template<typename T> ostream& operator<<(ostream& os, const unordered_set<T>& se){for(T x : se) os << x << " "; os << "\n"; return os;}
template<typename S, auto op, auto e> ostream& operator<<(ostream& os, const atcoder::segtree<S, op, e>& seg){int n = seg.max_right(0, [](S){return true;}); rep(i, n) os << seg.get(i) << (i == n - 1 ? "\n" : " "); return os;}
template<typename S, auto op, auto e, typename F, auto mapping, auto composition, auto id> ostream& operator<<(ostream& os, const atcoder::lazy_segtree<S, op, e, F, mapping, composition, id>& seg){int n = seg.max_right(0, [](S){return true;}); rep(i, n) os << seg.get(i) << (i == n - 1 ? "\n" : " "); return os;}

template<typename T> void chmin(T& a, T b){a = min(a, b);}
template<typename T> void chmax(T& a, T b){a = max(a, b);}

int main(){
	int n, m;
	cin >> n >> m;
	if(m % 2){
		cout << "-1\n";
		return 0;
	}
	string ans;
	rep(i, m) ans += 'R';
	vector<int> d(n);
	vector<pair<int, int>> G;
	rep(i, m){
		int u, v;
		cin >> u >> v;
		u--; v--;
		assert(u < v);
		if(u == 0 and v == n - 1){
			cout << "-1\n";
			return 0;
		}
		G.emplace_back(u, v);
		d[u]++;
		d[v]++;
	}
	int b = 0;
	if(d[0] > d[n - 1]){
		rep(i, m){
			if(b == m / 2) break;
			auto [u, v] = G[i];
			if(u == 0){
				ans[i] = 'B';
				b++;
			}
		}
		rep(i, m){
			if(b == m / 2) break;
			if(ans[i] == 'B') continue;
			auto [u, v] = G[i];
			if(v != n - 1){
				ans[i] = 'B';
				b++;
			}
		}
	}else{
		rep(i, m){
			if(b == m / 2) break;
			auto [u, v] = G[i];
			if(v == n - 1){
				ans[i] = 'B';
				b++;
			}
		}
		rep(i, m){
			if(b == m / 2) break;
			if(ans[i] == 'B') continue;
			auto [u, v] = G[i];
			if(u != 0){
				ans[i] = 'B';
				b++;
			}
		}
		
	}
	
	cout << ans << "\n";
	return 0;
}