#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
typedef long long ll;
const int INF = 1<<30;
const ll INFLL = 1LL<<60;
const ll MOD = 998244353;
const double INFD = 1.0E18;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, -1, 0, 1};
//const int dx[8] = {1, 1, 0, -1, -1, -1, 0, 1};
//const int dy[8] = {0, 1, 1, 1, 0, -1, -1, -1};
//const int dx[8] = {2, 1, -1, -2, -2, -1, 1, 2};
//const int dy[8] = {1, 2, 2, 1, -1, -2, -2, -1};
using Pair = pair<ll, ll>;
using Graph = vector<vector<pair<int, int>>>;
using mint = atcoder::modint998244353;


int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(15);
    int n, m; cin >> n >> m;
    Graph G(n);
    bool ok = true;
    for (int i = 0; i < m; i++){
        int u, v; cin >> u >> v; u--; v--;
        if (u > v) swap(u, v);
        if (u == 0 && v == n - 1) ok = false;
        G[u].push_back({v, i});
        G[v].push_back({u, i});
    }
    
    if (m % 2 == 1 || !ok){
        cout << -1 << endl;
        return 0;
    }
    
    vector<char> ans(m, 'B');
    int x = 0;
    if (G[0].size() > m / 2) x = n - 1;
    assert(G[x].size() <= m / 2);
    for (auto [_, idx] : G[x]) ans[idx] = 'R';
    int rem = m / 2 - G[x].size();
    int idx = 0;
    while (idx < m && rem > 0){
        if (ans[idx] == 'B'){
            ans[idx] = 'R';
            rem--;
        }
        idx++;
    }
    assert(rem == 0);
    
    for (int i = 0; i < m; i++) cout << ans[i];
    cout << '\n';
    
    return 0;
}