from sys import stdin,setrecursionlimit#,set_int_max_str_digits
#import pypyjit
#pypyjit.set_param('max_unroll_recursion=-1')
setrecursionlimit(20000000) # これこどふぉだと無理
#set_int_max_str_digits(200010)
mod = 998244353 
ii = lambda :int(stdin.readline())
mi = lambda :map(int,stdin.readline().split())
li = lambda :list(mi())
gmi = lambda :map(lambda x: int(x) - 1, stdin.readline().split())
gi = lambda :list(map(lambda x: 0 if x == "." else 1,input())) # グリッド入力受け取り
py = lambda :print("Yes")
pn = lambda :print("No")
pf = lambda :print("First") 
ps = lambda :print("Second")
vec = [(1,0),(-1,0),(0,-1),(0,1)]
vec1 = [(1,0),(1,1),(0,1),(-1,1),(-1,0),(-1,-1),(0,-1),(1,-1)] #8方向
inf = 2*10**18


from collections import defaultdict,deque

n,m = mi() 
g = defaultdict(list) 

if m % 2:
    print(-1) 
    exit() 

edges = [] 

memo1 = [] 
memo2 = [] 

for i in range(m):
    u,v = gmi() 
    if u >= v:
        u,v = v,u 
    if (u,v) == (0,n-1):
        print(-1) 
        exit()
    g[u].append(v) 
    g[v].append(u) 
    if u == 0:
        memo1.append((i,v)) 
    if v == n-1:
        memo2.append((i,u)) 
    edges.append((u,v)) 

seen1 = [0]*n # 頂点1から、頂点nを通らずに行ける頂点
que = deque() 
que.append(0) 

while que:
    v = que.popleft() 
    for u in g[v]:
        if seen1[u] == 0 and u != n-1:
            seen1[u] = 1 
            que.append(u) 

seen2 = [0]*n # 頂点nから、頂点1を通らずに行ける頂点
que = deque() 
que.append(n-1) 

while que:
    v = que.popleft() 
    for u in g[v]:
        if seen2[u] == 0 and u:
            seen2[u] = 1 
            que.append(u) 

nokori = m//2 
ans = [None]*m 

for i,v in memo2:
    if seen1[v]:
        ans[i] = "B" 

for i,v in memo1:
    if seen2[v]:
        ans[i] = "R" 
        nokori -= 1 
        assert nokori >= 0

for i in range(m):
    if ans[i] == None:
        if nokori:
            ans[i] = "R"
            nokori -= 1 
        else:
            ans[i] ="B"

print("".join(ans))