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

#define rep(i, l, r) for (int i = (int)(l); i<(int)(r); i++)
#define ll long long

int main() {
    int N, M; cin >> N >> M;
    if (M%2) {
        cout << -1 << endl;
        return 0;
    }
    vector<vector<pair<int, int>>> G(N);
    vector<int> unuse;
    rep(i, 0, M) {
        int u, v; cin >> u >> v;
        u--; v--;
        if (u == 0 and v == N-1) {
            cout << -1 << endl;
            return 0;
        }
        if (u != 0 and u != N-1 and v != 0 and v != N-1) {
            unuse.push_back(i);
        }
        G[u].push_back({v, i});
        G[v].push_back({u, i});
    }
    // cout << "ok1" << endl;
    auto reachable = [&](int v) -> vector<bool> {
        int x = N - 1 - v;
        vector<bool> ret(N, false);
        queue<int> Q;
        ret[v] = true;
        Q.push(v);
        while(!Q.empty()) {
            int u = Q.front(); Q.pop();
            // cout << u << " ";
            for (auto[to, id] : G[u]) {
                if (to == x) continue;
                if (!ret[to]) {
                    ret[to] = true;
                    Q.push(to);
                }
            }
        }
        // cout << endl;
        assert(ret[x] == false);
        return ret;
    };
    vector<bool> R1 = reachable(0), RN = reachable(N-1);
    // for (auto v : R1) cout << v << " ";
    // cout << endl;
    // for (auto v : RN) cout << v << " ";
    // cout << endl;
    string ans(M, '.');
    int R = 0, B = 0;
    for (auto[to, id] : G[0]) {
        if (!R1[to] or !RN[to]) {
            //いらない
            unuse.push_back(id);
        }
        else {
            ans[id] = 'R';
            R++;
        }
    }
    // cout << "ok" << endl;
    for (auto[to, id] : G[N-1]) {
        if (!R1[to] or !RN[to]) {
            //いらない
            unuse.push_back(id);
        }
        else {
            ans[id] = 'B';
            B++;
        }
    }
    // cout << "ok3" << endl;
    // cout << ans << endl;
    assert(R <= M/2);
    assert(B <= M/2);
    while(R < M/2) {
        int t = unuse.back(); unuse.pop_back();
        ans[t] = 'R';
        R++;
    }
    for (auto t : unuse) ans[t] = 'B';
    cout << ans << endl;
}