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

using ll = long long;

bool chmin(auto &a, auto b) { return a > b ? a = b, true : false; }
bool chmax(auto &a, auto b) { return a < b ? a = b, true : false; }

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int N, M;
    cin >> N >> M;
    vector<string> S(N);
    for (int i = 0; i < N; i++) cin >> S[i];
    vector<int> I(N);
    iota(I.begin(), I.end(), 0);
    string ans;
    for (int j = 0; j < M; j++) {
        int G = 0, C = 0, P = 0;
        for (int i : I) {
            G |= (S[i][j] == 'G');
            C |= (S[i][j] == 'C');
            P |= (S[i][j] == 'P');
        }
        int T = G + C + P;
        if (T == 3) {
            cout << -1 << endl;
            return 0;
        } else if (T == 2) {
            vector<int> J;
            if (!G) {
                ans.push_back('C');
                for (int i : I) {
                    if (S[i][j] == 'C') J.push_back(i);
                }
            } else if (!C) {
                ans.push_back('P');
                for (int i : I) {
                    if (S[i][j] == 'P') J.push_back(i);
                }
            } else {
                ans.push_back('G');
                for (int i : I) {
                    if (S[i][j] == 'G') J.push_back(i);
                }
            }
            I = J;
        } else if (T == 1) {
            if (G) {
                ans.push_back('P');
            } else if (C) {
                ans.push_back('G');
            } else {
                ans.push_back('C');
            }
            I.clear();
        } else {
            ans.push_back('G');
        }
    }
    cout << (I.size() ? "-1" : ans) << endl;
}