#include <bits/stdc++.h>
using namespace std;
using std::cerr;
using std::cin;
using std::cout;
#if __has_include(<atcoder/all>)
#include <atcoder/all>
using mint = atcoder::modint1000000007;
istream &operator>>(istream &is, mint &a) {
	int t;
	is >> t;
	a = t;
	return is;
}
ostream &operator<<(ostream &os, mint a) { return os << a.val(); }
#endif
#undef long
#define long long long
#define uint unsigned int
#define ull unsigned long
#define overload3(a, b, c, name, ...) name
#define rep3(i, a, b) for (int i = (a); i < (b); i++)
#define rep2(i, n) rep3(i, 0, n)
#define rep1(n) rep2(i, n)
#define rep(...) overload3(__VA_ARGS__, rep3, rep2, rep1)(__VA_ARGS__)
#define per3(i, a, b) for (int i = (b) - 1; i >= (a); i--)
#define per2(i, n) per3(i, 0, n)
#define per1(n) per2(i, n)
#define per(...) overload3(__VA_ARGS__, per3, per2, per1)(__VA_ARGS__)
#define ALL(a) a.begin(), a.end()
#define UNIQUE(a)                                                              \
	sort(ALL(a));                                                              \
	a.erase(unique(ALL(a)), a.end())
#define SZ(a) (int)a.size()
#define vec vector
#ifndef DEBUG
#define cerr                                                                   \
	if (false)                                                                 \
	cerr
#undef assert
#define assert(...) void(0)
#undef endl
#define endl '\n'
#endif
template <typename T> ostream &operator<<(ostream &os, vector<T> a) {
	const int n = a.size();
	rep(i, n) {
		os << a[i];
		if (i + 1 != n)
			os << " ";
	}
	return os;
}
template <typename T, size_t n>
ostream &operator<<(ostream &os, array<T, n> a) {
	rep(i, n) {
		os << a[i];
		if (i + 1 != n)
			os << " ";
	}
	return os;
}
template <typename T> istream &operator>>(istream &is, vector<T> &a) {
	for (T &i : a)
		is >> i;
	return is;
}
template <typename T, typename S> bool chmin(T &x, S y) {
	if ((T)y < x) {
		x = (T)y;
		return true;
	}
	return false;
}
template <typename T, typename S> bool chmax(T &x, S y) {
	if (x < (T)y) {
		x = (T)y;
		return true;
	}
	return false;
}
template <typename T> void operator++(vector<T> &a) {
	for (T &i : a)
		++i;
}
template <typename T> void operator--(vector<T> &a) {
	for (T &i : a)
		--i;
}
template <typename T> void operator++(vector<T> &a, int) {
	for (T &i : a)
		i++;
}
template <typename T> void operator--(vector<T> &a, int) {
	for (T &i : a)
		i--;
}
void solve() {
	int n, m;
	cin >> n >> m;
	vec<vec<pair<int, int>>> g(n);
	rep(i, m) {
		int u, v;
		cin >> u >> v;
		u--, v--;
		g[u].emplace_back(v, i);
		g[v].emplace_back(u, i);
	}
	vec<int> du(n, n), dv(n, n);
	queue<int> q;
	du[0] = 0;
	q.push(0);
	while (!q.empty()) {
		int x = q.front();
		q.pop();
		for (auto [i, idx] : g[x]) {
			if (du[i] != n)
				continue;
			du[i] = du[x] + 1;
			q.push(i);
		}
	}
	dv[n - 1] = 0;
	q.push(n - 1);
	while (!q.empty()) {
		int x = q.front();
		q.pop();
		for (auto [i, idx] : g[x]) {
			if (dv[i] != n)
				continue;
			dv[i] = dv[x] + 1;
			q.push(i);
		}
	}
	int c1 = 0, c2 = 0;
	vec<int> ans(m);
	for (auto [to, idx] : g[0]) {
		if (dv[0] == dv[to] + 1) {
			ans[idx] = 1;
			c1++;
		}
	}
	for (auto [to, idx] : g[n - 1]) {
		if (du[n - 1] == du[to] + 1) {
			ans[idx] = -1;
			c2++;
		}
	}
	rep(i, m) {
		if (ans[i] != 0)
			continue;
		if (2 * c1 < m) {
			ans[i] = 1;
			c1++;
		} else if (2 * c2 < m) {
			ans[i] = -1;
			c2++;
		}
	}
	if (c1 != c2) {
		cout << -1 << endl;
		return;
	}
	stringstream s;
	rep(i, m) {
		if (ans[i] == 1)
			s << 'R';
		if (ans[i] == -1)
			s << 'B';
	}
	string ss = s.str();
	if (ss.size() != m) {
		cout << -1 << endl;
		return;
	}
	cout << ss << endl;
	return;
}
int main() {
	// srand((unsigned)time(NULL));
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	cout << fixed << setprecision(20);
	int t = 1;
	// cin >> t;
	while (t--)
		solve();
}