#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic warning "-Wunused-function"
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  cin >> n >> m;
  vector<string> s(n);
  rep(i, n) cin >> s[i];
  string ans;
  VI is(n), nis;
  rep(i, n) is[i] = i;
  rep(j, m) {
    if (is.empty()) {
      ans += 'G';
      continue;
    }
    int seen = 0;
    for (int i : is) {
      char c = s[i][j];
      seen |= 1 << (c == 'G' ? 0 : c == 'C' ? 1 : 2);
    }
    if (seen == 7) {
      cout << -1 << '\n';
      return 0;
    }
    seen |= seen << 3;
    int b = 5;
    while (seen >> b & 1) b--;
    while (~seen >> b & 1) b--;
    b += 2;
    b %= 3;
    ans += "GCP"[b];
    nis.clear();
    for (int i : is) if (s[i][j] == "GCP"[b]) nis.emplace_back(i);
    swap(is, nis);
  }
  if (!is.empty()) ans = "-1";
  cout << ans << '\n';
}