#include <algorithm>
#include <atcoder/all>
#include <bitset>
#include <cassert>
#include <cmath>
#include <ctime>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;
using namespace atcoder;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repr(i, n) for (int i = (int)(n) - 1; i >= 0; i--)
#define repk(i, k, n) for (int i = k; i < (int)(n); i++)
#define all(v) v.begin(), v.end()
#define mod1 1000000007
#define mod2 998244353
#define mod3 100000007
#define vi vector<int>
#define vs vector<string>
#define vc vector<char>
#define vl vector<ll>
#define vb vector<bool>
#define vvi vector<vector<int>>
#define vvc vector<vector<char>>
#define vvl vector<vector<ll>>
#define vvb vector<vector<bool>>
#define vvvi vector<vector<vector<int>>>
#define vvvl vector<vector<vector<ll>>>
#define pii pair<int, int>
#define pil pair<int, ll>
#define pli pair<ll, int>
#define pll pair<ll, ll>
#define vpii vector<pair<int, int>>
#define vpll vector<pair<ll, ll>>
#define vvpii vector<vector<pair<int, int>>>
#define vvpll vector<vector<pair<ll, ll>>>

// using mint = modint998244353;

template <typename T>
void debug(T e) {
    cerr << e << endl;
}

template <typename T>
void debug(vector<T> &v) {
    rep(i, v.size()) { cerr << v[i] << " "; }
    cerr << endl;
}

template <typename T>
void debug(vector<vector<T>> &v) {
    rep(i, v.size()) {
        rep(j, v[i].size()) { cerr << v[i][j] << " "; }
        cerr << endl;
    }
}

template <typename T>
void debug(vector<pair<T, T>> &v) {
    rep(i, v.size()) { cerr << v[i].first << " " << v[i].second << endl; }
}

template <typename T>
void debug(set<T> &st) {
    for (auto itr = st.begin(); itr != st.end(); itr++) {
        cerr << *itr << " ";
    }
    cerr << endl;
}

template <typename T>
void debug(multiset<T> &ms) {
    for (auto itr = ms.begin(); itr != ms.end(); itr++) {
        cerr << *itr << " ";
    }
    cerr << endl;
}

template <typename T>
void debug(map<T, T> &mp) {
    for (auto itr = mp.begin(); itr != mp.end(); itr++) {
        cerr << itr->first << " " << itr->second << endl;
    }
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
    cerr << H << " ";
    debug_out(T...);
}

using mint = modint998244353;

vector<ll> dist(ll N, ll M, vector<ll> &u, vector<ll> &v, ll start) {
    vector<vector<ll>> graph(N, vector<ll>(0));
    for (ll i = 0; i < M; i++) {
        graph[u[i]].push_back(v[i]);
        graph[v[i]].push_back(u[i]);
    }
    queue<ll> que;
    vector<ll> dist(N, N + 1);
    que.push(start);
    dist[start] = 0;
    while (que.size()) {
        ll p = que.front();
        que.pop();
        for (ll x : graph[p]) {
            if (dist[x] < N) {
                continue;
            }
            dist[x] = dist[p] + 1;
            que.push(x);
        }
    }
    return dist;
}

string solve(ll N, ll M, vector<ll> &u, vector<ll> &v) {
    for (ll i = 0; i < M; i++) {
        u[i]--;
        v[i]--;
    }
    vector<vector<ll>> graph(N, vector<ll>(0));
    for (ll i = 0; i < M; i++) {
        graph[u[i]].push_back(v[i]);
        graph[v[i]].push_back(u[i]);
    }
    vector<char> cols(M);
    bool direct = false;
    for (ll i = 0; i < M; i++) {
        if (u[i] == 0 && v[i] == N - 1) {
            direct = true;
        }
    }
    if (direct) {
        return "-1";
    }
    vector<ll> e1(0);
    vector<ll> en(0);
    vector<bool> unlabel(M, true);
    for (ll i = 0; i < M; i++) {
        if (u[i] == 0) {
            e1.push_back(i);
            unlabel[i] = false;
        }
        if (v[i] == N - 1) {
            en.push_back(i);
            unlabel[i] = false;
        }
    }
    if (e1.size() <= M / 2 && en.size() <= M / 2) {
        for (ll i = 0; i < e1.size(); i++) {
            cols[e1[i]] = 'R';
        }
        ll rem_red = M / 2 - (ll)e1.size();
        for (ll i = 0; i < en.size(); i++) {
            cols[en[i]] = 'B';
        }
        ll rem_blu = M / 2 - (ll)en.size();
        for (ll i = 0; i < M; i++) {
            if (unlabel[i]) {
                if (rem_red > 0) {
                    cols[i] = 'R';
                    rem_red--;
                } else {
                    cols[i] = 'B';
                    rem_blu--;
                }
            }
        }
    } else if (e1.size() > M / 2) {
        ll rem_red = M / 2;
        ll rem_blu = M / 2;
        // 判断基準を次数ではなく、N
        // からの距離に変更して同じことをすればいける？
        vector<ll> d = dist(N, M, u, v, N - 1);
        for (ll i = 0; i < e1.size(); i++) {
            if (d[0] >= d[v[e1[i]]]) {
                cols[e1[i]] = 'R';
                rem_red--;
            }
        }
        // debug(cols);
        for (ll i = 0; i < e1.size(); i++) {
            if (d[0] < d[v[e1[i]]]) {
                if (rem_red > 0) {
                    cols[e1[i]] = 'R';
                    rem_red--;
                } else {
                    cols[e1[i]] = 'B';
                    rem_blu--;
                }
            }
        }
        assert(rem_red >= 0);
        for (ll i = 0; i < en.size(); i++) {
            cols[en[i]] = 'B';
        }
        for (ll i = 0; i < M; i++) {
            if (unlabel[i]) {
                cols[i] = 'B';
            }
        }
        ll red = 0;
        ll blue = 0;
        for (ll i = 0; i < M; i++) {
            if (cols[i] == 'R') {
                red++;
            } else if (cols[i] == 'B') {
                blue++;
            }
            // assert(cols[i] == 'R' || cols[i] == 'B');
        }
        // assert(red == M / 2 && blue == M / 2);
    } else {
        ll rem_red = M / 2;
        ll rem_blu = M / 2;
        vector<ll> d = dist(N, M, u, v, 0);
        for (ll i = 0; i < en.size(); i++) {
            if (d[N - 1] >= d[u[en[i]]]) {
                cols[en[i]] = 'R';
                rem_red--;
            }
        }
        for (ll i = 0; i < en.size(); i++) {
            if (d[N - 1] < d[u[en[i]]]) {
                if (rem_red > 0) {
                    cols[en[i]] = 'R';
                    rem_red--;
                } else {
                    cols[en[i]] = 'B';
                    rem_blu--;
                }
            }
        }
        for (ll i = 0; i < e1.size(); i++) {
            cols[e1[i]] = 'B';
        }
        for (ll i = 0; i < M; i++) {
            if (unlabel[i]) {
                cols[i] = 'B';
            }
        }
    }
    string ret = "";
    for (ll i = 0; i < M; i++) {
        ret += cols[i];
    }
    return ret;
}

int main() {
    ll N, M;
    cin >> N >> M;
    vector<ll> u(M);
    vector<ll> v(M);
    for (ll i = 0; i < M; i++) {
        cin >> u[i] >> v[i];
    }
    if (M % 2 == 1) {
        cout << -1 << endl;
        return 0;
    }
    cout << solve(N, M, u, v) << endl;
}