import sys
from operator import itemgetter, le
from collections import defaultdict, deque
import heapq
from heapq import heapify, heappop, _heapify_max, heappush
from bisect import bisect_left, bisect_right
import math
import itertools
import copy

stdin=sys.stdin
#sys.setrecursionlimit(10 ** 7)
## import pypyjit
## pypyjit.set_param('max_unroll_recursion=-1')

ip=lambda: int(sp())
fp=lambda: float(sp())
lp=lambda:list(map(int,stdin.readline().split()))
sp=lambda:stdin.readline().rstrip()
Yp=lambda:print('Yes')
Np=lambda:print('No')
inf = 1 << 60
mod = 10 ** 9 + 7
mod = 998244353
eps = 1e-9
sortkey1 = itemgetter(0)
sortkey2 = lambda x: (x[0], x[1])



###############################################################

N, K = lp()
A = []
col = [-1 for _ in range(N)]
RGB = 'RGB'
for _ in range(K):
    a, c = input().split()
    a = int(a) - 1;c = RGB.index(c)
    col[a] = c
    A.append((a,c))
A.sort(reverse=True)

rgb = [0 for _ in range(3)]
idx = N
ans = [-1 for _ in range(N)]
for i in range(N - 1, - 1, -1):
    if col[i] == -1:
        if ans[i] != -1:
            rgb[ans[i]] -= 1
        continue
    c = col[i]
    need = (i + 1) // 3 + 1
    r = min(i, idx) + 1
    if need - rgb[c] > r:
        print(-1)
        exit()
    t = 0
    use = need - rgb[c]
    for j in range(r, -1, -1):
        if t == use:
            break
        ans[r] = c
        rgb[c] += 1
        t += 1
        r -= 1
    #print(ans)
answer = []
for x in ans:
    answer.append(RGB[x])
print(''.join(answer))