import sys
input = sys.stdin.readline

N,M=map(int,input().split())

S=[input().strip() for i in range(N)]
X=dict()

X["P"]="C"
X["G"]="P"
X["C"]="G"

L=list(range(N))
Q=[([],L,0)]

while Q:
    Q.sort(key=lambda x:len(x[1]))
    now,L,ind=Q.pop()
    if L==[]:
        while len(now)<M:
            now.append("G")

        print("".join(now))
        exit()

    if ind==M:
        if L==[]:
            print("".join(now))
            exit()
        else:
            continue

    for x in "PGC":
        flag=1
        LL=[]
        for i in L:
            if S[i][ind]==X[x]:
                flag=0
                break
            if S[i][ind]==x:
                LL.append(i)

        if flag==1:
            Q.append((now+[x],LL,ind+1))
            
        


print(-1)