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

def f(T):
    if len(T) == 3:
        print(-1)
        exit()
    
    T = sorted(list(T))
    if len(T) == 0:
        return "C", "Z"
    
    elif len(T) == 1:
        if T[0] == "G":
            return "P", "G"
        elif T[0] == "C":
            return "G", "C"
        else:
            return "C", "P"
    
    else:
        if T == ["C", "G"]:
            return "G", "C"
        elif T == ["C", "P"]:
            return "C", "P"
        else:
            return "P", "G"

ans = []
nokori = set([i for i in range(N)])
for j in range(M):
    T = set()
    for i in nokori:
        T.add(S[i][j])
    
    dasu, kesu = f(T)
    ans.append(dasu)
    D = []
    for i in nokori:
        if S[i][j] == kesu:
            D.append(i)
    
    for d in D:
        nokori.remove(d)

if nokori:
    print(-1)
else:
    print("".join(ans))