#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 == 0 && v == n - 1) ok = false;
        G[u].push_back({v, i});
        G[v].push_back({u, i});
    }
    
    if (m % 2 == 1 || ok == false){
        cout << -1 << endl;
        return 0;
    }
    
    atcoder::dsu uf(n);
    vector<char> ans(m, 'B');
    int x = 0;
    if (G[0].size() > m / 2) x = n - 1;
    queue<int> q;
    for (auto [v, idx] : G[x]){
        ans[idx] = 'R';
        q.push(v);
        uf.merge(x, v);
    }
    int rem = m / 2 - G[x].size();
    
    while (!q.empty()){
        if (rem == 0) break;
        int v = q.front(); q.pop();
        for (auto [nv, idx] : G[v]){
            if (nv == n - 1 - x) continue;
            if (ans[idx] == 'R') continue;
            uf.merge(v, nv);
            ans[idx] = 'R';
            rem--;
            if (rem == 0) break;
            q.push(nv);
        }
    }
    
    queue<int> q2; q2.push(n - 1 - x);
    while (!q2.empty()){
        if (rem == 0) break;
        int v = q2.front(); q2.pop();
        for (auto [nv, idx] : G[v]){
            if (uf.same(nv, x)) continue;
            if (ans[idx] == 'R') continue;
            ans[idx] = 'R';
            rem--;
            if (rem == 0) break;
            q2.push(nv);
        }
    }
    
    for (int i = 0; i < m; i++) cout << ans[i];
    cout << endl;
    
    return 0;
}