from cmath import inf


n,k = map(int,input().split())
ans = []
RGB = [0]*3

AC = []
for i in range(k):
    a,c = input().split()
    a = int(a)
    AC.append([a,c])
AC.sort()
l = "RGB"
now = 0
for i in range(1,n+1):
    
    target = 0
    ind = -1
    if now < k and AC[now][0] == i:
        c = AC[now][1]
        ind = l.index(c)

        if max(RGB) == RGB[ind]:
            if RGB.count(max(RGB)) == 3:
                target = ind
            else:
                for j in range(3):
                    if RGB[j] != max(RGB):
                        target = j
                        break

        else:
            target = ind


        now += 1
    else:
        for j in range(3):
            if RGB[j] != max(RGB):
                target = j
                break
    
    ans.append(target)
    RGB[target] += 1

    assert max(RGB)-min(RGB) <= 1

    if ind != -1 and max(RGB) != RGB[ind]:
        print(-1)
        exit()
        
ans = [l[i] for i in ans]
print("".join(ans))