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

from collections import deque

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)]
    queue = deque()
    passed = [False] * N
    if len(next_nodes[0]) < len(next_nodes[N - 1]):
        passed[0] = True
        queue.append(0)
        f1 = N - 1
    else:
        passed[N - 1] = True
        queue.append(N - 1)
        f1 = 0

    r_num = M // 2
    b_num = M // 2
    while len(queue) > 0:
        v = queue.popleft()
        for w, i in next_nodes[v]:
            if answers[i] is None:
                if b_num > 0 and w == f1:
                    b_num -= 1
                    answers[i] = "B"
                else:
                    r_num -= 1
                    answers[i] = "R"
                if r_num == 0:
                    break
            
            if w != f1:
                if not passed[w]:
                    passed[w] = True
                    queue.append(w)
        if r_num == 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()