#include <bits/stdc++.h>
using namespace std;

#define rep(i, a, b) for (int i = a; i < b; i++)
using ll = long long;

int main(){
	int n, m;
	cin >> n >> m;
	vector<string> s(n);
	rep(i, 0, n) cin >> s[i];
	map<char, int> mp;
	mp['G'] = 0;
	mp['C'] = 1;
	mp['P'] = 2;
	string t;
	vector<int> v(n);
	rep(j, 0, m) {
		vector<int> cnt(3);
		rep(i, 0, n) {
			if(v[i]) continue;
			cnt[mp[s[i][j]]]++;
		}
		if(cnt[0] && cnt[1] && cnt[2]) {
			cout << -1 << endl;
			return 0;
		}
		if(cnt[1] == 0 || (cnt[1] == 0 && cnt[2] == 0)) t += 'P';
		else if(cnt[2] == 0 || (cnt[2] == 0 && cnt[0] == 0)) t += 'G';
		else if(cnt[0] == 0 || (cnt[0] == 0 && cnt[1] == 0)) t += 'C';
		rep(i, 0, n) {
			if(v[i]) continue;
			if(s[i][j] == 'G' && t.back() == 'P') v[i] = 1;
			if(s[i][j] == 'P' && t.back() == 'C') v[i] = 1;
			if(s[i][j] == 'C' && t.back() == 'G') v[i] = 1;
		}
	}
	rep(i, 0, n) {
		if(v[i] == 0) {
			cout << -1 << endl;
			return 0;
		}
	}
	cout << t << endl;
}