N, M = (int(x) for x in input().split())
mem=[]
for i in range(N):
    S=str(input())
    mem.append(S)

ans = []
ok = set()
for i in range(M):
    S=set()
    for j in range(N):
        if j not in ok:
            S.add(mem[j][i])

    if len(S) == 1:
        a = S.pop()
        if a == "G":
            ans.append("P")
        elif a == "C":
            ans.append("G")
        else:
            ans.append("C")
        for i in range(M-i-1):
            ans.append("G")
        print("".join(ans))
        exit()

    elif len(S) == 3:
        print(-1)
        exit()

    else:
        if "G" in S and "P" in S:
            ans.append("P")
            for j in range(N):
                if mem[j][i] == "G":
                    ok.add(j)
        elif "C" in S and "P" in S:
            ans.append("C")
            for j in range(N):
                if mem[j][i] == "P":
                    ok.add(j)
        else:
            ans.append("G")
            for j in range(N):
                if mem[j][i] == "C":
                    ok.add(j)

    if len(ok) == N:
        for i in range(M-i-1):
            ans.append("G")
        print("".join(ans))
        exit()

if len(ok) == N:
    print("".join(ans))
else:
    print(-1)