#include <atcoder/all>
#include <bits/stdc++.h>
#define rep(i, a, b) for (ll i = (ll)(a); i < (ll)(b); i++)
using namespace atcoder;
using namespace std;

typedef long long ll;

template <typename T> ostream &operator<<(ostream &os, const vector<T> &v) {
    int n = v.size();
    rep(i, 0, n) { os << v[i] << " \n"[i == n - 1]; }
    return os;
}

int main() {
    cin.tie(0);
    cout.tie(0);
    ios::sync_with_stdio(0);
    int n, m;
    cin >> n >> m;
    if (m % 2) {
        cout << "-1\n";
        return 0;
    }
    vector<int> u(m), v(m);
    rep(i, 0, m) {
        cin >> u[i] >> v[i], u[i]--, v[i]--;
        if (u[i] == 0 && v[i] == n - 1) {
            cout << "-1\n";
            return 0;
        }
    }

    auto f = [&](int s, int t) {
        set<int> st;
        dsu uf(n);
        rep(i, 0, m) {
            if (u[i] == s || v[i] == s) {
                continue;
            }
            uf.merge(u[i], v[i]);
        }
        rep(i, 0, n) {
            if (s == i)
                continue;
            if (uf.same(i, t)) {
                st.insert(i);
            }
        }
        vector<int> ret;
        rep(i, 0, m) {
            if (u[i] == s && st.count(v[i])) {
                ret.emplace_back(v[i]);
            }
            swap(u[i], v[i]);
            if (u[i] == s && st.count(v[i])) {
                ret.emplace_back(v[i]);
            }
            swap(u[i], v[i]);
        }
        return ret;
    };

    auto v1 = f(0, n - 1);
    auto vn = f(n - 1, 0);

    map<pair<int, int>, char> ma;
    for (auto x : v1) {
        ma[{0, x}] = 'B';
    }
    for (auto x : vn) {
        ma[{x, n - 1}] = 'R';
    }
    int B = m / 2, R = m / 2;
    string ans(m, '.');
    rep(i, 0, m) {
        if (ma.count({u[i], v[i]})) {
            ans[i] = ma[{u[i], v[i]}];
            if (ans[i] == 'R')
                R--;
            if (ans[i] == 'B')
                B--;
        }
    }
    rep(i, 0, m) {
        if (ans[i] == '.') {
            if (R >= B) {
                R--;
                ans[i] = 'R';
            } else {
                B--;
                ans[i] = 'B';
            }
        }
    }
    cout << ans << endl;
}