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

class UnionFind:
    def __init__(self, n):
        self._tree = [-1]*n
        # number of connected component
        self.cnt = n

    def root(self, u):
        stack = []
        while self._tree[u] >= 0:
            stack.append(u)
            u = self._tree[u]
        for v in stack: self._tree[v] = u
        return u

    def same(self, u, v):
        return self.root(u) == self.root(v)

    def merge(self, u, v):
        u, v = self.root(u), self.root(v)
        if u == v: return False
        if self._tree[u] > self._tree[v]: u, v = v, u
        self._tree[u] += self._tree[v]
        self._tree[v] = u
        self.cnt -= 1
        return True

    # size of connected component
    def size(self, u):
        return -self._tree[self.root(u)]


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

for v,i in to[0]:
    if v==n-1:
        print(-1)
        exit()

if m&1:
    print(-1)
    exit()

cnt=[m//2,m//2]
ans=[-1]*m
root = 0
c = 0
uf = UnionFind(n)
for u, v in uv:
    if u == root or v == root: continue
    uf.merge(u, v)
for v, i in to[root]:
    if uf.same(v, n-1-root):
        ans[i] = c
        cnt[c] -= 1

root = n-1
c = 1
uf = UnionFind(n)
for u, v in uv:
    if u == root or v == root: continue
    uf.merge(u, v)
for v, i in to[root]:
    if uf.same(v, n-1-root):
        ans[i] = c
        cnt[c] -= 1

for i in range(m):
    if ans[i]==-1:
        if cnt[0]:
            ans[i]=0
            cnt[0]-=1
        else:
            ans[i]=1

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