#include <bits/stdc++.h>

// #include <atcoder/fenwicktree.hpp>
// #include <atcoder/segtree.hpp>
// #include <atcoder/modint.hpp>
// #include <atcoder/dsu.hpp>
// #include <atcoder/lazysegtree.hpp>

// using namespace atcoder;
using namespace std;
using ll = long long;
using ull = unsigned long long;
template <class T>
using max_heap = priority_queue<T>;
template <class T>
using min_heap = priority_queue<T, vector<T>, greater<>>;
ll ll_min = numeric_limits<ll>::min();
ll ll_max = numeric_limits<ll>::max();
ll ALPHABET_N = 26;
// using mint = modint998244353;
#define rep(i, n) for (ll i = (ll)0; i < (ll)n; i++)
#define rep_(i, k, n) for (ll i = (ll)k; i < (ll)n; i++)
#define all(a) a.begin(), a.end()

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	ll n, m;
	cin >> n >> m;
	vector<string> S(n);
	rep(i, n) cin >> S[i];
	string ans;
	rep(i, m)
	{
		set<char> st;
		rep(j, n)
		{
			st.insert(S[j][i]);
		}
		if (st.size() == 1)
		{
			char c = *st.begin();
			if (c == 'G')
				ans += 'P';
			if (c == 'C')
				ans += 'G';
			if (c == 'P')
				ans += 'C';
		}
		else if (st.size() == 2)
		{
			if (st == set<char>({'P', 'G'}))
				ans += 'P';
			if (st == set<char>({'P', 'C'}))
				ans += 'C';
			if (st == set<char>({'G', 'C'}))
				ans += 'G';
		}
		else
		{
			ans = "-1";
			break;
		}
	}
	cout << ans << endl;
	return 0;
}