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

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

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

    if x==0 and y==N-1:
        print(-1)
        exit()
    if x==N-1 and y==0:
        print(-1)
        exit()

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


if len(E[0])<M//2:
    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")==ANS.count("B"):
        print("".join(ANS))
        exit()

if len(E[N-1])<M//2:
    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")==ANS.count("B"):
        print("".join(ANS))
        exit()


print(-1)