import sys
import pypyjit
import itertools
import heapq
import math
from collections import deque, defaultdict
import bisect
import string
import re

input = sys.stdin.readline
sys.setrecursionlimit(10 ** 6)
pypyjit.set_param('max_unroll_recursion=-1')


def index_lt(a, x):
    'return largest index s.t. A[i] < x or -1 if it does not exist'
    return bisect.bisect_left(a, x) - 1


def index_le(a, x):
    'return largest index s.t. A[i] <= x or -1 if it does not exist'
    return bisect.bisect_right(a, x) - 1


def index_gt(a, x):
    'return smallest index s.t. A[i] > x or len(a) if it does not exist'
    return bisect.bisect_right(a, x)


def index_ge(a, x):
    'return smallest index s.t. A[i] >= x or len(a) if it does not exist'
    return bisect.bisect_left(a, x)


N, K = map(int, input().split())
query = []
for _ in range(K):
    a, c = input().split()
    query.append((int(a), c))
query.sort(key=lambda x: x[0])

ans = [None] * N
cur = 0
cnt = {"R": 0, "G": 0, "B": 0}
for a, c in query:
    while cnt[c] < -(-a // 3):
        ans[cur] = c
        cnt[c] += 1
        cur += 1
    if cur > a:
        print(-1)
        exit()
for i in range(N):
    if ans[i] is None:
        ans[i] = "R"
print("".join(ans))