N, M = map(int, input().split())
S = [input() for _ in range(N)]

A = ["G", "C", "P"]

ans = []
F = [False]*N
for j in range(M):
    SET = set()
    for i in range(N):
        if not F[i]:
            SET.add(S[i][j])
    if len(SET) == 1:
        ans.append(A[(A.index(SET.pop())-1)%3])
    elif len(SET) == 2:
        if "G" not in SET:
            ans.append("C")
        elif "C" not in SET:
            ans.append("P")
        else:
            ans.append("G")
    elif len(SET) == 3:
        exit(print(-1))
    else:
        break
    for i in range(N):
        if (A.index(ans[-1])+1)%3 == A.index(S[i][j]):
            F[i] = True
while len(ans) < M:
    ans.append("G")

print("".join(ans) if sum(F) == N else -1)