import sys

# sys.setrecursionlimit(200005)
# sys.set_int_max_str_digits(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

# dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
# inf = -1-(-1 << 31)
inf = -1-(-1 << 62)

# md = 10**9+7
md = 998244353

n,m=LI()
to=[[] for _ in range(n)]
uv=[]
uv2i={}
for i in range(m):
    u,v=LI1()
    to[u].append(v)
    to[v].append(u)
    uv.append((u,v))
    uv2i[u,v]=uv2i[v,u]=i

if m&1 or 0 in to[n-1] or len(to[0])*2>m or len(to[n-1])*2>m:
    print(-1)
    exit()

c=[m//2,m//2]
ans=[-1]*m
for v in to[0]:
    i=uv2i[0,v]
    ans[i]=0
    c[0]-=1
for v in to[n-1]:
    i=uv2i[v,n-1]
    ans[i]=1
    c[1]-=1
for i in range(m):
    if ans[i]==-1:
        if c[0]:
            ans[i]=0
            c[0]-=1
        else:
            ans[i]=1

print("".join("RB"[c] for c in ans))