#include <bits/stdc++.h>

//    #include <boost/multiprecision/cpp_int.hpp>
 #define int long long
 #define inf  1000000007
 #define pa pair<int,int>
 #define ll long long
 #define pal pair<double,double>
 #define ppap pair<pa,int>
  #define PI 3.14159265358979323846
  #define paa pair<int,char>
  #define  mp make_pair
  #define  pb push_back
  #define EPS (1e-10)
                                          
    int dx[8]={0,1,0,-1,1,1,-1,-1};
    int dy[8]={1,0,-1,0,-1,1,1,-1};
                                            using namespace std;
                                   			class pa3{
                                            	public:
                                            	int x;
                                   				int y,z;
                                            	pa3(int x=0,int y=0,int z=0):x(x),y(y),z(z) {}
                                            	bool operator < (const pa3 &p) const{
                                            		if(x!=p.x) return x<p.x;
                                            		if(y!=p.y) return y<p.y;
                                            		 return z<p.z;
                                            		//return x != p.x ? x<p.x: y<p.y;
                                            	}
                                   				bool operator > (const pa3 &p) const{
                                            		if(x!=p.x) return x>p.x;
                                            		if(y!=p.y) return y>p.y;
                                            		 return z>p.z;
                                            		//return x != p.x ? x<p.x: y<p.y;
                                            	}
                                            	bool operator == (const pa3 &p) const{
                                            		return x==p.x && y==p.y && z==p.z;
                                            	}
                                            		bool operator != (const pa3 &p) const{
                                            			return !( x==p.x && y==p.y && z==p.z);
                                            	}
                                            
                                            };
                                            
                                            class pa4{
                                            	public:
                                            	int x;
                                            	int y,z,w;
                                            	pa4(int x=0,int y=0,int z=0,int w=0):x(x),y(y),z(z),w(w) {}
                                            	bool operator < (const pa4 &p) const{
                                            		if(x!=p.x) return x<p.x;
                                            		if(y!=p.y) return y<p.y;
                                            		if(z!=p.z)return z<p.z;
                                            		return w<p.w;
                                            		//return x != p.x ? x<p.x: y<p.y;
                                            	}
                                            	bool operator > (const pa4 &p) const{
                                            		if(x!=p.x) return x>p.x;
                                            		if(y!=p.y) return y>p.y;
                                            		if(z!=p.z)return z>p.z;
                                            		return w>p.w;
                                            		//return x != p.x ? x<p.x: y<p.y;
                                            	}
                                            	bool operator == (const pa4 &p) const{
                                            		return x==p.x && y==p.y && z==p.z &&w==p.w;
                                            	}
                                            		
                                            
                                            };
                                            class pa2{
                                            	public:
                                            	int x,y;
                                            	pa2(int x=0,int y=0):x(x),y(y) {}
                                            	pa2 operator + (pa2 p) {return pa2(x+p.x,y+p.y);}
                                            	pa2 operator - (pa2 p) {return pa2(x-p.x,y-p.y);}
                                            	bool operator < (const pa2 &p) const{
                                            		return y != p.y ? y<p.y: x<p.x;
                                            	}
                                            	bool operator > (const pa2 &p) const{
                                            		return x != p.x ? x<p.x: y<p.y;
                                            	}
                                            	bool operator == (const pa2 &p) const{
                                            		return abs(x-p.x)==0 && abs(y-p.y)==0;
                                            	}
                                            	bool operator != (const pa2 &p) const{
                                            		return !(abs(x-p.x)==0 && abs(y-p.y)==0);
                                            	}
                                            		
                                            
                                            };
                                            

                                            class Point{
                                            	public:
                                            	double x,y;
                                            	Point(double x=0,double y=0):x(x),y(y) {}
                                            	Point operator + (Point p) {return Point(x+p.x,y+p.y);}
                                            	Point operator - (Point p) {return Point(x-p.x,y-p.y);}
                                            	Point operator * (double a) {return Point(x*a,y*a);}
                                            	Point operator / (double a) {return Point(x/a,y/a);}
                                            	double absv() {return sqrt(norm());}
                                            	double norm() {return x*x+y*y;}
                                            	bool operator < (const Point &p) const{
                                            		return x != p.x ? x<p.x: y<p.y;
                                            	}
                                            	bool operator == (const Point &p) const{
                                            		return fabs(x-p.x)<EPS && fabs(y-p.y)<EPS;
                                            	}
                                            };
                                            typedef Point Vector;
                                     #define pl pair<int,pas>
                                            struct Segment{
                                            Point p1,p2;
                                            };
                                             double dot(Vector a,Vector b){
                                            	return a.x*b.x+a.y*b.y;
                                            }
                                            double cross(Vector a,Vector b){
                                            	return a.x*b.y-a.y*b.x;
                                            }
                                        
                bool parareru(Point a,Point b,Point c,Point d){
                //	if(abs(cross(a-b,d-c))<EPS)cout<<"dd "<<cross(a-b,d-c)<<endl;
                	return abs(cross(a-b,d-c))<EPS;
                }
                double distance_ls_p(Point a, Point b, Point c) {
                  if ( dot(b-a, c-a) < EPS ) return (c-a).absv();
                  if ( dot(a-b, c-b) < EPS ) return (c-b).absv();
                  return abs(cross(b-a, c-a)) / (b-a).absv();
                }
                bool is_intersected_ls(Segment a,Segment b) {
                	if(a.p1==b.p1||a.p2==b.p1||a.p1==b.p2||a.p2==b.p2) return false;
                	if(parareru((a.p2),(a.p1),(a.p1),(b.p2))&&parareru((a.p2),(a.p1),(a.p1),(b.p1))){
                //		cout<<"sss"<<endl;
                		if(dot(a.p1-b.p1,a.p1-b.p2)<EPS) return true;
                		if(dot(a.p2-b.p1,a.p2-b.p2)<EPS) return true;
                		if(dot(a.p1-b.p1,a.p2-b.p1)<EPS) return true;
                		if(dot(a.p1-b.p2,a.p2-b.p2)<EPS) return true;
                		return false;
                	}
                  else return ( cross(a.p2-a.p1, b.p1-a.p1) * cross(a.p2-a.p1, b.p2-a.p1) < EPS ) && ( cross(b.p2-b.p1, a.p1-b.p1) * cross(b.p2-b.p1, a.p2-b.p1) < EPS );
                }
                 
                double segment_dis(Segment a,Segment b){
                	if(is_intersected_ls(a,b))return 0;
                	double r=distance_ls_p(a.p1, a.p2, b.p1);
                	r=min(r,distance_ls_p(a.p1, a.p2, b.p2));
                	r=min(r,distance_ls_p(b.p1, b.p2, a.p2));
                	r=min(r,distance_ls_p(b.p1, b.p2, a.p1));
                	return r;
                }
                Point intersection_ls(Segment a, Segment b) {
                  Point ba = b.p2-b.p1;
                  double d1 = abs(cross(ba, a.p1-b.p1));
                  double d2 = abs(cross(ba, a.p2-b.p1));
                  double t = d1 / (d1 + d2);
                 
                  return a.p1 + (a.p2-a.p1) * t;
                }
                
                                string itos( int i ) {
                                ostringstream s ;
                                s << i ;
                                return s.str() ;
                                }
                                 
                                int gcd(int v,int b){
                                	if(v>b) return gcd(b,v);
                                	if(v==b) return b;
                                	if(b%v==0) return v;
                                	return gcd(v,b%v);
                                }
                 
                                double distans(double x1,double y1,double x2,double y2){
                                	double rr=(x1-x2)*(x1-x2)+(y1-y2)*(y1-y2);
                                	return sqrt(rr);
                                	
                                }
                                int mod;
int extgcd(int a, int b, int &x, int &y) {
    if (b == 0) {
        x = 1;
        y = 0;
        return a;
    }
    int d = extgcd(b, a%b, y, x);
    y -= a/b * x;
    return d;
}
pa operator+(const pa & l,const pa & r) {   
    return {l.first+r.first,l.second+r.second};                                    
}    
pa operator-(const pa & l,const pa & r) {   
    return {l.first-r.first,l.second-r.second};                                    
}  
                                
                int pr[1200010];
                int inv[1200010];
                
                int beki(int wa,int rr,int warukazu){
                	if(rr==0) return 1%warukazu;
                	if(rr==1) return wa%warukazu;
                	wa%=warukazu;
                	if(rr%2==1) return ((ll)beki(wa,rr-1,warukazu)*(ll)wa)%warukazu;
                	ll zx=beki(wa,rr/2,warukazu);
                	return (zx*zx)%warukazu;
                }
    double bekid(double w,int r){
    	if(r==0) return 1.0;
    	if(r==1) return w;
    	if(r%2) return bekid(w,r-1)*w;
    	double f=bekid(w,r/2);
    	return f*f;
    }
                
    			int comb(int nn,int rr){
    				int r=pr[nn]*inv[rr];
    				r%=mod;
    				r*=inv[nn-rr];
    				r%=mod;
    				return r;
    			}
                
                void gya(int ert){
                	pr[0]=1;
                	for(int i=1;i<=ert;i++){
                		pr[i]=(pr[i-1]*i)%mod;
                	}
                	for(int i=0;i<=ert;i++) inv[i]=beki(pr[i],mod-2,mod);
                	
                }
                
              //   cin.tie(0);
    		//	ios::sync_with_stdio(false);
    			//priority_queue<pa3,vector<pa3>,greater<pa3>> pq;            
                 //sort(ve.begin(),ve.end(),greater<int>());
    
       
       
                               //----------------kokomade tenpure------------
// verify

// SPOJ sarray
// JAG 茂木地区2017 Separate String
// コドフェス2016トーナメント1B 数字列をカンマで分ける問題
// ARC050 D
// square869120Contest #2 部分文字列

struct SAIS{
	private:
	public:
/*
SA  LCP
10    0   $
 9    1   a$
 5    1   aabba$
 0    2   ababbaabba$
 6    4   abba$
 2    0   abbaabba$
 8    2   ba$
 4    2   baabba$
 1    1   babbaabba$
 7    3   bba$
 3    0** bbaabba$
	
	SAとrankを作るときは
	make_suffix_array_string(string s);
	(rankはSAの逆写像)
	
	
	LCPもいるときは
	make_suffix_array_string
	のあとで
	make_lcp
	
	
*/
	string STR;
	vector<int> S;
	vector<int> SL;
	// L-type = 0
	// S-type = 1
	// LMS-type =2
	vector<pa> haba;
	vector<int> SA;
	vector<int> rank;
	vector<int> LCP;
	int mojisu;
	int len;
	
	void make_suffix_array_string(string str){
		STR=str;
		set<char> setc;
		map<char,int> mapc;
		for(auto c:str)setc.insert(c);
		int cnt=1;
		for(auto it=setc.begin();it!=setc.end();it++){
			mapc[*it]=cnt;
			cnt++;
		}
		for(auto c:str)S.pb(mapc[c]);
		mojisu=cnt;
		S.pb(0);
		make_suffix_array_naibu();
	}
	
	void make_suffix_array_saiki(vector<int> ve,int max_valu){
		S=ve;
		S.pb(0);
		mojisu=max_valu+1;
		make_suffix_array_naibu();
	}
	
	
	void make_suffix_array_naibu(){
		int cnt=mojisu;
		len=S.size();
		
		vector<int> kosuu(cnt,0);
		for(auto c:S)kosuu[c]++;
		
		haba.pb({0,0});
		for(int i=1;i<cnt;i++){
			haba.pb({haba.back().second+1,haba.back().second+kosuu[i]});
		}
		
		///////////////
		//S,L,LMSを判定
		// L-type   = 0
		// S-type   = 1
		// LMS-type <0: LMS-substrの長さ＊－１
		///////////////

		SL.resize(len);
		for(int i=len-1;i>=0;i--){
			if(i==len-1){
				SL[i]=1;
			}
			else{
				if(S[i]<S[i+1]) SL[i]=1;
				else if(S[i]>S[i+1])SL[i]=0;
				else{
					SL[i]=SL[i+1];
				}
			}
		}
		
		int LMS_num=0;
		vector<int> LMS;
		vector<int> LMS_id(len,0);
		int rt=0;
		for(int i=1;i<len;i++)if(SL[i-1]==0 &&SL[i]==1){
			LMS.pb(i);
			LMS_id[i]=rt;
			LMS_num++;
			rt++;
		}
		for(int i=0;i<LMS_num-1;i++){
			SL[LMS[i]]=-(LMS[i+1]-LMS[i]+1);
		}
		SL[len-1]=-1;
		/*
		for(int i=0;i<len;i++)cout<<SL[i]<<" ";
		cout<<endl;
		*/
		for(int i=0;i<len;i++)SA.pb(-1);
		vector<int> pos(cnt,0);
		for(int c=0;c<len;c++)if(SL[c]<0){
			
			SA[haba[S[c]].second-pos[S[c]]]=c;
			pos[S[c]]++;
		}
		///////////////
		// 
		// 
		// 最初のinduced sort
		// 
		///////////////
		
		
		induced_sort(SA,len,cnt);
		
	//	cout<<"d"<<endl;
		///////////////
		// 
		// 
		// LMSの重複チェック
		// 
		///////////////
		
		vector<pa> lms_sub_order;
		int mae_ind;
		for(int i=0;i<len;i++)if(SL[SA[i]]<0){
			if(i==0){
				lms_sub_order.pb({SA[i],1});
				
			}
			else if(lms_sub_order.back().second==1){
				lms_sub_order.pb({SA[i],2});
			}
			else if(SL[SA[mae_ind]]!=SL[SA[i]]){
				lms_sub_order.pb({SA[i],lms_sub_order.back().second+1});
			}
			else{
				int lmslen= -SL[SA[mae_ind]];
				bool bo=true;
				for(int j=0;j<lmslen;j++){
					if(S[SA[mae_ind]+j]!=S[SA[i]+j]){
						bo=0;
						break;
					}
				}
				if(bo)lms_sub_order.pb({SA[i],lms_sub_order.back().second});
				else lms_sub_order.pb({SA[i],lms_sub_order.back().second+1});
			}
			mae_ind=i;
		}
	//	for(auto v:lms_sub_order)cout<<v.first<<" "<<v.second<<endl;
	//	cout<<endl;
			for(int i=0;i<cnt;i++)pos[i]=0;
			for(int i=0;i<len;i++)SA[i]=-1;
		
		if(LMS_num==lms_sub_order.back().second){
			for(int i=LMS_num-1;i>=0;i--){
				int c=lms_sub_order[i].first;
				SA[haba[S[c]].second-pos[S[c]]]=c;
				pos[S[c]]++;
			}
	
		}
		else{
			vector<int> rec((int)LMS.size());
			for(int i=0;i<LMS_num;i++){
				rec[LMS_id[lms_sub_order[i].first]]=lms_sub_order[i].second;
			}
			SAIS ss_naibu;
			ss_naibu.make_suffix_array_saiki(rec,lms_sub_order.back().second);
			
			for(int i=LMS_num;i>0;i--){
				int c=LMS[ss_naibu.SA[i]];
				SA[haba[S[c]].second-pos[S[c]]]=c;
				pos[S[c]]++;
			}
		}

			induced_sort(SA,len,cnt);
		rank.resize(len);
		for(int i=0;i<len;i++)rank[SA[i]]=i;
			return;
	}
	
	void induced_sort(vector<int> &ve, int len,int cnt){
		vector<int> ue(cnt,0);
		for(int i=0;i<len;i++){
			if(ve[i]==-1) continue;
			if(ve[i]==0) continue;
			if(SL[ve[i]-1]==0){
				ve[haba[S[ve[i]-1]].first+ue[S[ve[i]-1]]]=ve[i]-1;
				ue[S[ve[i]-1]]++;
			}
			if(SL[ve[i]]<0 && i>0) ve[i]=-1;
		}
		for(int i=0;i<cnt;i++)ue[i]=0;
		
		for(int i=len-1;i>=0;i--){
			if(ve[i]==-1) continue;
			if(ve[i]==0) continue;
			if(SL[ve[i]-1]!=0){
				ve[haba[S[ve[i]-1]].second-ue[S[ve[i]-1]]]=ve[i]-1;
				ue[S[ve[i]-1]]++;
			}
		}
		for(auto v:ve)if(v==-1)assert(0);
	}
	
	void make_lcp(){
		LCP.resize(len);
		int h=0;
		LCP[0]=0;
		for(int i=0;i<len-1;i++){
			int j=SA[rank[i]-1];
			if(h>0)h--;
			for(;j+h<len-1&& i+h<len-1;h++){
				if(S[j+h]!=S[i+h])break;
			}
			LCP[rank[i]-1]=h;
		}
	//	for(int i=0;i<len;i++)cout<<LCP[i]<<endl;
		return;
	}
	
	int lower_bound(string p){// 文字列の場合のみ　pは非空
		int m=p.length();
		int ue=len,si=0,me;
		while(ue-si>1){
			me=(ue+si)/2;
			if(STR.compare(SA[me],m,p)<0)si=me;
			else ue=me;
		}
		return ue;
	}
	
	bool exist(string p){
		int l=p.length();
		int id=lower_bound(p);
		
	//	cout<<STR.compare(SA[id],l,p)<<endl;
		return 0==STR.compare(SA[id],l,p);
	}
	pa range(string p){// pを含む半壊区間 [l,r)  l==r なら存在なし
				return {lower_bound(p),lower_bound(p+'{')};
	}
};
SAIS SA;


string ima="";

void dfs(int len){
	if(len==0){
		
		if(!SA.exist(ima)){
			cout<<ima<<endl;
			exit(0);
		}
		return;
	}
	for(char c='a';c<='z';c++){
		ima.pb(c);
		dfs(len-1);
		ima.pop_back();
		
	}
}
 signed main(){
 	

    	       cin.tie(0);
   			ios::sync_with_stdio(false);
 	string s;
 	
 	int n;
 	cin>>n;
 	
 	for(int i=0;i<n;i++){
 		string t;
 		cin>>t;
 		t.pb('#');
 		s+=t;
 	}
 	SA. make_suffix_array_string(s);
 	
 	for(int len=1;;len++){
 		ima="";
 		dfs(len);
 	
 	}
 	
 	return 0;
  }


Measured memory_bytes: 41176