#include <bits/stdc++.h>
#include <atcoder/dsu>
using namespace std;

int main() {
    int n, m;
    cin >> n >> m;
    if (m % 2 == 1) {
        cout << -1 << endl;
        return 0;
    }

    vector<vector<pair<int, int>>> g(n);
    vector<int> e(m, -1);
    atcoder::dsu uf1(n), uf2(n);

    for (int i = 0; i < m; i++) {
        int a, b;
        cin >> a >> b;
        if (a == 1 && b == n) {
            cout << -1 << endl;
            return 0;
        }
        --a; --b;
        g[a].emplace_back(b, i);
        g[b].emplace_back(a, i);
        if (a != 0 && b != n - 1) {
            uf1.merge(a, b);
            uf2.merge(a, b);
        }
    }

    for (const auto& x : g[0]) {
        uf1.merge(g[0][0].first, x.first);
    }
    for (const auto& x : g[n - 1]) {
        uf2.merge(g[n - 1][0].first, x.first);
    }
    
    for (const auto& x : g[0]) {
        if (uf2.same(x.first, g[n - 1][0].first)) {
            e[x.second] = 0;
        }
    }
    for (const auto& x : g[n - 1]) {
        if (uf1.same(x.first, g[0][0].first)) {
            e[x.second] = 1;
        }
    }
    
    if (count(e.begin(), e.end(), 0) > m / 2 || count(e.begin(), e.end(), 1) > m / 2) {
        cout << -1 << endl;
        return 0;
    }

    int dif = m / 2 - count(e.begin(), e.end(), 0);
    for (int i = 0; i < m; i++) {
        if (e[i] == -1) {
            if (dif != 0) {
                dif--;
                e[i] = 0;
            } else {
                e[i] = 1;
            }
        }
    }
    
    string ans;
    for (int x : e) {
        ans += (x == 0 ? 'B' : 'R');
    }
    cout << ans << endl;
    
    return 0;
}