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