#pragma GCC optimize("Ofast")
#pragma GCC target("avx2")

#define rd_init() char*rp=({char*mmap();mmap(0l,1l<<25,1,2,0,0ll);})
#define rd() ({int _v=0,_c;while(_c=*rp++-48,_c>=0)_v=_v*10+_c;_v;})
#define rep(v,e) for(typeof(e) v=0;v<e;++v)

#define HASH_BITS 16
#define HASH_MASK ((1<<HASH_BITS)-1)
unsigned hash[1<<HASH_BITS];

int hash_add(unsigned x){
	int h=x&HASH_MASK;
	while(1){
		if(hash[h]==0){
			hash[h]=x;
			return h;
		}
		if(hash[h]==x){
			return h;
		}
		h=h+1&HASH_MASK;
	}
}

int hn[1<<HASH_BITS];
char ss[1000][11];
unsigned ch[26];

void f0(){
	unsigned v=1;
	rep(i,26){
		ch[i]=v;
		v*=257;
	}
}

void f1(){
	rd_init();
	int n=rd();
	rep(i,n){
		char*s=ss[i];
		int j=0;
		unsigned v=0;
		for(int c;c=*rp++,c>='a';){
			s[j++]=c;
			v+=ch[c-'a'];
		}
		rep(c,26){
			int h=hash_add(v+ch[c]);
			hn[h]=hn[h]?-1:i+1;
		}
	}
}

void f2(){
	rep(h,1<<HASH_BITS){
		if(hn[h]>0){
			char wbuf[16];
			memcpy(wbuf,ss[hn[h]-1],11);
			unsigned v=hash[h];
			int j=0;
			for(int c;c=wbuf[j++],c>='a';){
				v-=ch[c-'a'];
			}
			rep(c,26){
				if(ch[c]==v){
					wbuf[j-1]=c+'a';
					write(1,wbuf,j);
					_exit(0);
				}
			}
			_exit(1);
		}
	}
	write(1,"-1",2);
	_exit(0);
}

int main(){
	f0();
	f1();
	f2();
}