import sys
input = sys.stdin.readline
N, M = map(int, input().split())
S = [input().strip() for _ in range(N)]
ans = []
S = set(S)
janken = ["G", "C", "P"]
jan = {"G": 0, "C": 1, "P": 2}
for i in range(M):
    tmp = set()
    G = set()
    C = set()
    P = set()
    if len(S)==0:
        break
    for j in S:
        tmp.add(j[i])
        if j[i]=="G":
            G.add(j)
        elif j[i]=="C":
            C.add(j)
        else:
            P.add(j)
    tmp = list(tmp)
    if len(tmp)==1:
        t = tmp[0]
        a = janken[(jan[t]-1)%3]
        ans.append(a)
        b = janken[(jan[a]+1)%3]
    elif len(tmp)==2:
        t0, t1 = jan[tmp[0]], jan[tmp[1]]
        if (t0-1)%3==t1:
            a = janken[t1]
            ans.append(a)
            b = janken[(jan[a]+1)%3]
        elif (t1-1)%3==t0:
            a = janken[t0]
            ans.append(a)
            b = janken[(jan[a]+1)%3]
    else:
        print(-1)
        exit()

    if b=="G":
        for g in G:
            S.discard(g)
    elif b=="C":
        for c in C:
            S.discard(c)
    else:
        for p in P:
            S.discard(p)

if len(S)>0:
    print(-1)
    exit()
while len(ans)<M:
    ans.append("G")
print("".join(ans))