#ifdef NACHIA #define _GLIBCXX_DEBUG #else #define NDEBUG #endif #include #include #include #include namespace nachia { struct DsuFast{ private: std::vector w; public: DsuFast(int n = 0) : w(n, -1) {} int leader(int u){ if(w[u] < 0) return u; return w[u] = leader(w[u]); } int operator[](int u){ return leader(u); } int merge(int u, int v){ u = leader(u); v = leader(v); if(u == v) return u; if(-w[u] < -w[v]) std::swap(u, v); w[u] += w[v]; w[v] = u; return u; } int size(int u){ return -w[leader(u)]; } bool same(int u, int v){ return leader(u) == leader(v); } }; } // namespace nachia using i64 = long long; using u64 = unsigned long long; #define rep(i,n) for(i64 i=0; i void chmin(A& l, const A& r){ if(r < l) l = r; } template void chmax(A& l, const A& r){ if(l < r) l = r; } using namespace std; void testcase(){ i64 N, M; cin >> N >> M; bool ok = true; vector U(M), V(M), D(N); rep(i,M){ i64 u,v; cin >> u >> v; u--; v--; if(u == 0 && v == N-1) ok = false; U[i] = u; V[i] = v; D[u]++; D[v]++; } if(!ok || M%2 == 1){ cout << "-1\n"; return; } if(D[0] > M/2){ for(auto& u : U) u = N-1-u; for(auto& u : V) u = N-1-u; swap(U, V); } auto dsu = nachia::DsuFast(N); string ans = string(M, 'R'); i64 cnt = 0; auto check = [&](i64 i){ if(cnt == M/2) return; cnt++; ans[i] = 'B'; dsu.merge(U[i], V[i]); }; rep(i,M) if(U[i] == 0) check(i); rep(i,M) if(ans[i] != 'B' && V[i] != N-1) check(i); rep(i,M) if(V[i] == N-1 && !dsu.same(0, U[i])) check(i); cout << ans << "\n"; } int main(){ ios::sync_with_stdio(false); cin.tie(nullptr); rep(t,1) testcase(); return 0; }