import sys
input = sys.stdin.readline

N,M=map(int,input().split())

EDGE=[list(map(int,input().split())) for i in range(M)]

for i in range(M):
    EDGE[i][0]-=1
    EDGE[i][1]-=1

if M%2==1:
    print(-1)
    exit()

E=[[] for i in range(N)]

for i in range(M):
    x,y=EDGE[i]

    E[x].append((y,i))
    E[y].append((x,i))

ANS=["R"]*M
NOW=0
Q=[0]
USE=[0]*N
USE[0]=1

while Q:
    x=Q.pop()
    for to,ind in E[x]:
        if to==N-1:
            continue
        if ANS[ind]=="R" and NOW<M//2:
            ANS[ind]="B"
            if USE[to]==0:
                Q.append(to)
                USE[to]=1
            NOW+=1

if ANS.count("R")==M//2:
    print("".join(ANS))
    exit()

ANS=["R"]*M
NOW=0
Q=[N-1]
USE=[0]*N
USE[N-1]=1

while Q:
    x=Q.pop()
    for to,ind in E[x]:
        if to==0:
            continue
        if ANS[ind]=="R" and NOW<M//2:
            ANS[ind]="B"
            if USE[to]==0:
                Q.append(to)
                USE[to]=1
            NOW+=1

if ANS.count("R")==M//2:
    print("".join(ANS))
    exit()

print(-1)