#include<stdio.h>
char s[100005], ss[100005];
int id[100005], len;
char ans[100003];
int main()
{
	int n, m;
	scanf("%d %d", &n, &m);
	int i, j, k;
	for (i = 0; i < n; i++)
	{
		scanf("%s", ss);
		for (j = 0; j < m; j++)
			s[m * i + j] = ss[j];
	}
	for (i = 0; i < n; i++)
		id[i] = i;
	len = n;
	for (i = 0; i < m; i++)
		ans[i] = '?';
	ans[m] = '\0';
	int cnt[4];
	char q[4] = "GCP";
	for (j = 0; j < m; j++)
	{
		cnt[0] = cnt[1] = cnt[2] = 0;
		for (i = 0; i < len; i++)
		{
			if (s[m * id[i] + j] == 'G')
				cnt[0]++;
			else if (s[m * id[i] + j] == 'C')
				cnt[1]++;
			else
				cnt[2]++;
		}
		if (cnt[0] > 0 && cnt[1] > 0 && cnt[2] > 0)
		{
			printf("-1\n");
			return 0;
		}
		cnt[3] = cnt[0];
		for (i = 0; i < 3; i++)
		{
			if (cnt[i] > 0 && cnt[i + 1] > 0)
			{
				ans[j] = q[i];
				k = 0;
				for (i = 0; i < len; i++)
					if (s[m * id[i] + j] == ans[j])
						id[k++] = id[i];
				len = k;
				break;
			}
		}
		if (ans[j] != '?')
			continue;
		for (i = 0; i < 3; i++)
			if (cnt[i] > 0)
				ans[j] = q[(i + 2) % 3];
		for (j++; j < m; j++)
			ans[j] = 'G';
		printf("%s\n", ans);
		return 0;
	}
	printf("-1\n");
	return 0;
}