import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################


from collections import defaultdict
 
class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n
 
    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]
 
    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)
 
        if x == y:
            return
 
        if self.parents[x] > self.parents[y]:
            x, y = y, x
 
        self.parents[x] += self.parents[y]
        self.parents[y] = x
 
    def size(self, x):
        return -self.parents[self.find(x)]
 
    def same(self, x, y):
        return self.find(x) == self.find(y)
 
    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]
 
    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]
 
    def group_count(self):
        return len(self.roots())
 
    def all_group_members(self):
        group_members = defaultdict(list)
        for member in range(self.n):
            group_members[self.find(member)].append(member)
        return group_members
 
    def __str__(self):
        return '\n'.join(f'{r}: {m}' for r, m in self.all_group_members().items())
n, m = na()
if m % 2:
    print(-1)
    exit()

E = []
for _ in range(m):
    u, v = na()
    u -= 1
    v -= 1
    if u == 0 and v == n -1:
        print(-1)
        exit()
    E.append((u, v))

uf = UnionFind(n)
for u, v in E:
    if u != 0:
        uf.union(u, v)

ans = [-1] * m
f = 0
t = 0
for u, v in E:
    if u == 0 and uf.same(v, n - 1):
        ans[t] = 1
        f += 1
    t += 1

uf = UnionFind(n)
for u, v in E:
    if v != n-1:
        uf.union(u, v)
t = 0
for u, v in E:
    if v == n - 1 and uf.same(0, u):
        ans[t] = 0
        f -= 1
    t += 1

# print(ans)
for t in range(m):
    u, v = E[t]
    if ans[t] == -1:
        if f > 0:
            ans[t] = 0
            f -= 1
        else:
            ans[t] = 1
            f += 1

print("".join(["RB"[ans[i]] for i in range(m)]))