# https://yukicoder.me/problems/no/3056


def main():
    N, M = map(int, input().split())
    edges = []
    for _ in range(M):
        u, v = map(int, input().split())
        edges.append((u - 1, v - 1))
    
    if M % 2 == 1:
        print(-1)
        return
    
    next_nodes = [[] for _ in range(N)]
    for i in range(M):
        u, v = edges[i]
        next_nodes[u].append((v, i))
        next_nodes[v].append((u, i))

    for x, _ in next_nodes[0]:
        if x == N - 1:
            print(-1)
            return
        
    answers = [None for _ in range(M)]
    r_num = M // 2
    b_num = M // 2
    f1 = 0
    f2 = N -1 
    if len(next_nodes[0]) < len(next_nodes[v]):
        f1 = 0
        f2 = N -1
    else:
        f1 = N -1
        f2 = 0

    m = len(next_nodes[f1])
    for _, i in next_nodes[f1]:
        answers[i] = "R"
        r_num -= 1
    for _ , i in next_nodes[f2]:
        answers[i] = "B"            
        m -= 1
        b_num -= 1
        if m == 0:
            break
    
    for i in range(M):
        if answers[i] is None:
            if r_num > 0:
                answers[i] = "R"
                r_num -= 1
            elif b_num > 0:
                answers[i] = "B"
                b_num -= 1
    
    print("".join(answers))
    

    





if __name__ == "__main__":
    main()