import sys

# sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = (1 << 63)-1
# inf = (1 << 31)-1
# md = 10**9+7
md = 998244353

s = "RGB"
n, k = LI()
color = [set() for _ in range(n)]
nxt = [-1]*n
ac = []
for _ in range(k):
    a, c = SI().split()
    a = int(a)-1
    c = s.find(c)
    color[a].add(c)
    nxt[a] = c
    ac.append((a, c))

for i in range(n-1)[::-1]:
    color[i] |= color[i+1]
    if nxt[i] == -1: nxt[i] = nxt[i+1]
# print(color)
# print(nxt)

aa = [-1]*n
cnt = [0, 0, 0]
for i in range(n):
    nc = nxt[i]
    if cnt[nc] != max(cnt):
        aa[i] = nc
    else:
        mnc, mn = 0, inf
        for c in color[i]:
            if cnt[c] < mn: mn, mnc = cnt[c], c
        if mn == cnt[nc]:
            aa[i] = nc
        else:
            aa[i] = mnc
    cnt[aa[i]] += 1

ac.sort(reverse=True)
cnt = [0, 0, 0]
for i, a in enumerate(aa):
    cnt[a] += 1
    if ac and ac[-1][0] == i:
        _, c = ac.pop()
        if cnt[c] != max(cnt):
            print(-1)
            exit()

print("".join(s[i] for i in aa))