from itertools import product

N, K = map(int, input().split())

testimonials = []
for _ in range(K):
    Ai, ci = input().split()
    Ai = int(Ai)
    testimonials.append((Ai, ci))
possible_colors = ['R', 'G', 'B']
all_possible_hats = product(possible_colors, repeat=N)
def is_valid_configuration(hats):
    for Ai, ci in testimonials:
        color_count = {'R': 0, 'G': 0, 'B': 0}
        for i in range(Ai):
            color_count[hats[i]] += 1
        max_color_count = max(color_count.values())
        if color_count[ci] != max_color_count:
            return False
    
    return True
for hats in all_possible_hats:
    if is_valid_configuration(hats):
        print(''.join(hats))
        break
else:
    print(-1)