#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 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, m; cin >> n >> m;
	vector to(n, vector<P>());
	rep(i, m) {
		int u, v; cin >> u >> v;
		u--, v--;
		to[u].push_back({ v,i });
		to[v].push_back({ u,i });
	}
	if (m % 2 == 1) {
		cout << -1 << endl;
		return 0;
	}
	vector<int>col0(m, -1);
	{
		set<int>st = { 0 };
		queue<int>q;
		q.push(0);
		while (!q.empty()) {
			auto v = q.front();
			q.pop();
			for (auto[nv, idx] : to[v]) {
				if (nv == n - 1)continue;
				if (col0[idx] == -1) {
					col0[idx] = 0;
				}
				if (st.count(nv))continue;
				st.insert(nv);
				q.push(nv);
			}
		}
	}
	vector<int>col1(m, -1);
	{
		set<int>st = { n - 1 };
		queue<int>q;
		q.push(n - 1);
		while (!q.empty()) {
			auto v = q.front();
			q.pop();
			for (auto[nv, idx] : to[v]) {
				if (nv == 0)continue;
				if (col1[idx] == -1) {
					col1[idx] = 0;
				}
				if (st.count(nv))continue;
				st.insert(nv);
				q.push(nv);
			}
		}
	}
	rep(i, m) {
		if (col0[i] == -1 && col1[i] == -1) {
			cout << -1 << endl;
			return 0;
		}
	}
	int c0 = 0, c1 = 0;
	vector<int>col(m, -1);
	rep(i, m) {
		if (col0[i] == -1) {
			col[i] = 1;
			c1++;
		}
		else if (col1[i] == -1) {
			col[i] = 0;
			c0++;
		}
	}
	if (c0 > m / 2 || c1 > m / 2) {
		cout << -1 << endl;
		return 0;
	}
	rep(i, m) {
		if (col[i] == -1) {
			if (c0 < m / 2) {
				col[i] = 0;
				c0++;
			}
			else {
				col[i] = 1;
				c1++;
			}
		}
	}
	string ans;
	rep(i, m) {
		if (col[i])ans += 'R';
		else ans += 'B';
	}
	cout << ans << endl;
	return 0;
}