N, M = map(int,input().split())
data = []
for i in range(N):
    data.append(input())
li = [True] * N
ans = []
for i in range(M):
    s = set()
    for j in range(N):
        if(li[j]):
            s.add(data[j][i])
    if(len(s) == 3):
        print(-1)
        exit()
    if(len(s) == 1):
        s = list(s)
        if(s[0] == "G"):
           a = "P"
        if(s[0] == "C"):
           a = "G"
        if(s[0] == "P"):
           a = "C"

        ans.append(a)
        for i in range(M -i - 1):
            ans.append("G")
        print("".join(ans))
        exit()
    if(len(s) == 2):
        b = sorted(list(s))
        if(b[0] == "C" and b[1] == "P"):
            a = "P"
            c = "C"
        if(b[0] == "C" and b[1] == "G"):
            a = "C"
            c = "G"
        if(b[0] == "G" and b[1] == "P"):
            a = "G"
            c = "P"
        ans.append(c)
        for j in range(N):
            if(li[j] and data[j][i] == a):
                li[j] = False
print(-1)