#include <bits/stdc++.h>
using namespace std;
typedef signed long long ll;

#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define FORR2(x,y,arr) for(auto& [x,y]:arr)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
template<class T> bool chmax(T &a, const T &b) { if(a<b){a=b;return 1;}return 0;}
template<class T> bool chmin(T &a, const T &b) { if(a>b){a=b;return 1;}return 0;}
//-------------------------------------------------------

int N,M;
string V[101010];

void solve() {
	int i,j,k,l,r,x,y; string s;
	
	cin>>N>>M;
	set<int> avail;
	FOR(i,N) {
		cin>>V[i];
		avail.insert(i);
	}
	string W;
	FOR(i,M) {
		set<char> S;
		FORR(a,avail) S.insert(V[a][i]);
		if(S.size()==0) {
			W+="C";
		}
		else if(S.size()==1) {
			if(*S.begin()=='G') W+="P";
			if(*S.begin()=='C') W+="G";
			if(*S.begin()=='P') W+="C";
			avail.clear();
		}
		else if(S.size()==2) {
			if(*S.begin()=='C'&&*S.rbegin()=='G') {
				W+="G";
			}
			if(*S.begin()=='C'&&*S.rbegin()=='P') {
				W+="C";
			}
			if(*S.begin()=='G'&&*S.rbegin()=='P') {
				W+="P";
			}
			set<int> a2;
			FORR(a,avail) if(V[a][i]==W[i]) a2.insert(a);
			avail=a2;
		}
		else if(S.size()==3) {
			cout<<-1<<endl;
			return;
		}
	}
	cout<<W<<endl;
	
	
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false), cin.tie(0);
	FOR(i,argc-1) s+=argv[i+1],s+='\n'; FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	cout.tie(0); solve(); return 0;
}