N, M = map(int, input().split())
S = [input() for _ in range(N)]
fin = set()
H = "GCPG"

def next_h(c):
    return H[H.index(c) + 1]

ans = []
for k in range(M):
    hand_tmp = {'G', 'C', 'P'}
    for i in range(N):
        if i in fin:
            continue
        hand_tmp.discard(next_h(S[i][k]))
    match len(hand_tmp):
        case 0:
            print(-1)
            exit()
        case 1:
            hand = hand_tmp.pop()
        case 2:
            hand = next_h([h for h in H if h not in hand_tmp].pop())
    for i in range(N):
        if S[i][k] == next_h(hand):
            fin.add(i)
    ans.append(hand)
if len(fin) == N:
    print(*ans, sep='')
else:
    print(-1)