import sys
input = lambda:sys.stdin.readline().rstrip()
sys.setrecursionlimit(1007)

def scc(n, edge):
  redge=[[]for _ in range(n)]
  for i in range(n):
    for j in edge[i]:
      redge[j].append(i)
  order=[]
  used=[0]*n
  group=[-1]*n
  def dfs(u):
    used[u]=1
    for v in edge[u]:
      if not used[u]:
        dfs(v)
    order.append(u)
  def rdfs(u,c):
    group[u]=c
    used[u]=1
    for v in redge[u]:
      if not used[v]:
        rdfs(v,c)
  for i in range(n):
    if not used[i]:
      dfs(i)
  used=[0]*n
  x=0
  for u in order[::-1]:
    if not used[u]:
      rdfs(u,x)
      x+=1
  return x,group

def bipartite_graph_check(n, edge):
  color = [-1] * n
  for i in range(n):
    if color[i] != -1:
      continue
    stack = [i]
    color[i] = 0
    for node in stack:
      for mode in edge[node]:
        if color[mode] == -1:
          stack.append(mode)
          color[mode] = color[node] ^ 1
        elif color[mode] == color[node]:
          return False
  return True

n,m=map(int,input().split())
edge=[[] for _ in range(n)]
redge=[[] for _ in range(n)]
edges=[[-1]*n for _ in range(n)]
for i in range(m):
  a,b=map(int,input().split())
  a-=1
  b-=1
  redge[b].append(a)
  edge[a].append(b)
  edges[a][b]=i

if bipartite_graph_check(n,edge):
  exit(print(-1))
label,_=scc(n,edge)
if label!=1:
  exit(print(-1))

mix=n+1
node=[]
for i in range(n):
  root=[-1]*n
  stack=[i]
  flag=False
  nodee=[]
  for u in stack:
    for v in edge[u]:
      if i==v:
        nodee.append(u)
        while root[u]!=-1:
          u=root[u]
          nodee.append(u)
        break
      if root[v]!=-1:continue
      root[v]=u
      stack.append(v)
    if len(nodee):
      break
  if len(nodee)<mix:
    mix=len(nodee)
    node=nodee[::-1]
if mix==n:
  exit(print(-1))
nodeout=[(len(edge[i]),i)for i in node]
nodeout.sort(reverse=1)
ind=node.index(nodeout[0][1])
node=node[ind:]+node[:ind]
ans=["R"]*m
node+=node
for i in range(mix//2+1):
  ans[edges[node[i]][node[i+1]]]="G"
print("".join(ans))