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:
    if not used[u]:
      rdfs(u,x)
      x+=1
  return x,group

n,m=map(int,input().split())
edge=[[] 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
  edge[a].append(b)
  edges[a][b]=i

label,_=scc(n,edge)
if label!=1:
  exit(print(-1))
x=[]
for i in range(n):
  used=[0]*n
  co=[0]*n
  stack=[i]
  flag=False
  for u in stack:
    for v in edge[u]:
      if used[v]:continue
      used[v]=1
      co[v]=co[u]+1
      if i==v:
        flag=True
        break
      stack.append(v)
    if flag:
      break
  x.append(co[i])
s=x.index(min(x))
stack=[s]
root=[-1]*n
for u in stack:
  for v in edge[u]:
    if s==v:
      x=[u]
      while root[u]!=-1:
        u=root[u]
        x.append(u)
      ans=["R"]*m
      x=x[::-1]
      if len(x)==n:
        x.append(x[0])
        for i in range(n):
          if i%2==0:
            ans[edge[x[i]][x[i+1]]]="G"
      else:
        for i in range(len(x)//2+1):
          ans[edges[x[i]][x[i+1]]]="G"
      print("".join(ans))
      exit()
    if root[v]!=-1:
      continue
    root[v]=u
    stack.append(v)