from collections import deque

N, M = map(int, input().split())
G = [[] for _ in range(N)]
for i in range(M):
    u, v = map(int, input().split())
    u, v = u-1, v-1
    G[u].append((v, i))
    G[v].append((u, i))

if M%2 == 1:
    exit(print(-1))
if N-1 in G[0]:
    exit(print(-1))

visited = [False]*N
visited[0] = True
que = deque()
que.append(0)
ST = set()
while que:
    n = que.popleft()
    for v, idx in G[n]:
        if visited[v]:
            continue
        if v != N-1:
            visited[v] = True
            que.append(v)
        else:
            ST.add(idx)
visited = [False]*N
visited[N-1] = True
que = deque()
que.append(N-1)
SS = set()
while que:
    n = que.popleft()
    for v, idx in G[n]:
        if visited[v]:
            continue
        if v != 0:
            visited[v] = True
            que.append(v)
        else:
            SS.add(idx)

red, blue = M//2, M//2
ans = [-1]*M
for n in SS:
    ans[n] = 0
    red -= 1
for n in ST:
    ans[n] = 1
    blue -= 1
for i in range(M):
    if ans[i] != -1:
        continue
    if red:
        ans[i] = 0
        red -= 1
    else:
        ans[i] = 1
        blue -= 1

print("".join(["R" if ans[i] == 0 else "B" for i in range(M)]))