import sys,random,bisect
from collections import deque,defaultdict
from heapq import heapify,heappop,heappush
from itertools import permutations
from math import log,gcd

input = lambda :sys.stdin.readline().rstrip()
mi = lambda :map(int,input().split())
li = lambda :list(mi())

N,M = mi()
edge = [[] for i in range(N)]
for _ in range(M):
    a,b = mi()
    edge[a-1].append(b-1)

for s in range(N):
    visit = [[False,False] for i in range(N)]
    visit[s][0] = True
    stack = [(s,0)]
    while stack:
        v,p = stack.pop()
        for nv in edge[v]:
            if not visit[nv][1-p]:
                visit[nv][1-p] = True
                stack.append((nv,1-p))
    if any(not visit[i][0] for i in range(N)):
        exit(print(-1))

ans = ["R"] * (M//2) + ["G"] * (M//2)
if M%2:
    ans += ["R"]

p = [-1 for i in range(N)]
for i in range(N):
    if len(edge[i])==1:
        p[i] = edge[i][0]
    else:
        break
else:
    exit(print(-1))

random.shuffle(ans)
print("".join(ans))