package main

import . "fmt"

func main() {
	var n,m int
	Scan(&n,&m)
	ss := make([][]byte, n)
	for i := range ss {
		var s string
		Scan(&s)
		ss[i] = []byte(s)
	}
	ok := make([]bool, n)
	ans := make([]byte, m)
	const (
		G = 1
		C = 2
		P =4
	)
	for i := range ans {
		f := 0
		for j, s := range ss {
			if ok[j] {
				continue
			}
			switch s[i] {
				case 'G':
					f |= G
				case 'C':
					f |= C
				case 'P':
					f |= P
			}
		}
		switch f {
			case 0, C, (G|C):
				ans[i] = 'G'
			case P, (C|P):
				ans[i] = 'C'
			case G, (G|P):
				ans[i] = 'P'
			case (G|C|P):
				Println(-1)
				return
		}
		for j, s := range ss {
			if ok[j] {
				continue
			}
			if s[i] != ans[i] {
				ok[j] = true
			}
		}
	}
	Println(string(ans))
}