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

#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(v) v.begin(),v.end()
#define RALL(v) v.rbegin(),v.rend()
template <class T> using V=vector<T>;
template <class T> using VV=V<V<T>>;
using u128=__int128_t;
using ll=long long;
 
int main(){
  ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  
  ll n,m;
  cin>>n>>m;
  V<string> S(n);
  rep(i,n) cin>>S[i];
  
  V<char> A(m,'G');
  V<int> D(n);
  rep(j,m){
    int g=0,c=0,p=0;
    rep(i,n){
      if(D[i]) continue;
      if(S[i][j]=='G') g=1;
      else if(S[i][j]=='C') c=1;
      else p=1;
    }
    if(g+c+p==3){
      cout<<-1<<endl;
      return 0;
    }
    else if(g+c==2){
      rep(i,n) if(S[i][j]=='C') D[i]=1;
    }
    else if(c+p==2){
      A[j]='C';
      rep(i,n) if(S[i][j]=='P') D[i]=1;
    }
    else if(p+g==2){
      A[j]='P';
      rep(i,n) if(S[i][j]=='G') D[i]=1;
    }
    else if(g==1){
      A[j]='P';
      rep(i,n) if(S[i][j]=='G') D[i]=1;
      break;
    }
    else if(c==1){
      rep(i,n) if(S[i][j]=='C') D[i]=1;
      break;
    }
    else if(p==1){
      A[j]='C';
      rep(i,n) if(S[i][j]=='P') D[i]=1;
      break;
    }
    else break;
  }
  bool b=true;
  rep(i,n){
    if(D[i]==0){
      cout<<-1<<endl;
      return 0;
    }
  }
  rep(j,m) cout<<A[j];
  cout<<endl;
    
  return 0;
}