N, K = map(int, input().split())
AC = []
for _ in range(K):
    a, c = input().split()
    AC.append((int(a), c))
AC.sort()
ans = []
dic = {"R": 0, "G": 1, "B": 2}
color = ["R", "G", "B"]
X = [0, 0, 0]
used = 0
for a, c in AC:
    unused = a - used
    i = dic[c]
    xnow = X[i]
    xmax = max(X)
    xdif = xmax - xnow
    if unused < xdif:
        print(-1)
        exit()
    else:
        X[i] += xdif
        ans += [c] * xdif
        unused -= xdif
        used += xdif
    Y = X[0:]
    Y.sort()
    x2nd = Y[1]
    xdif2 = xmax - x2nd
    if unused < xdif2:
        continue
    else:
        xadd = - ((- unused + xdif2) // 2)
        X[i] += xadd
        ans += [c] * xadd
        used += xadd
ans += ["R"] * (N - used)
print("".join(ans))