//sub-BOF
#define _AOJ_
        /*vvv>
        zzzzzI
 .---.  zzuzuI                 .vgggg&,.
+++++=  dAC:|I  .WbbWo       JMM9^```?TMB`  ..&gNNg,.   gggggggJ,   qgggggggg] (&&&&&&&&[   c+OA&J,   (&&&&&&+J,   .cJeAA&-.  (&&&&&&&&x   .&AA&=-.
+++++=  dTqk|I  Xpbpbp      JM#`           (M#^   ?MMp  MM|   +TMN. JMF      ' |yk      ` dVY    7Vk,  Vy     XV  cVf     ?Y!  JM         V$      `
+++++=  dcf:|I  Xppppp      dMN           .MM+     .MM  MM|     MM] JMMMMMM+   |@tqkoh)  ,y0      (V$  yyyyyyyV7  VV           JMWyZWr    TWVVVVW&,
++++++  d7qk|0  Xppppp      ^HMN,    _.db  WMm,   .MMF  MM|   ..MM` JMF      . |yk       .WV&.   .XW'  yy   4yn.  jyn      +.  JM                #S
`++++`  ?ZZZX=  ?WWWW=        -THMMMMH9^    (TMMMMM9!   MMMMMMM""   JMMMMMMMME |UU.        ?TUUUUY=    UU.   (UU-  ^7TUUUV7!   JUUUUUUUU  7TUNKO*/


//basic
#pragma GCC target("sse4")
#include "bits/stdc++.h"
using namespace std;
typedef long long lint;
typedef long double ld;
typedef string cs;
#define all(v) v.begin(),v.end()
#define pb push_back

//rep
#define _vcppunko4(tuple) _getname4 tuple
#define _getname4(_1,_2,_3,_4,name,...) name
#define _getname3(_1,_2,_3,name,...) name
#define _trep2(tuple) _rep2 tuple
#define _trep3(tuple) _rep3 tuple
#define _trep4(tuple) _rep4 tuple
#define _rep1(n) for(lint i=0;i<n;++i)
#define _rep2(i,n) for(lint i=0;i<n;++i)
#define _rep3(i,a,b) for(lint i=a;i<b;++i)
#define _rep4(i,a,b,c) for(lint i=a;i<b;i+=c)
#define _trrep2(tuple) _rrep2 tuple
#define _trrep3(tuple) _rrep3 tuple
#define _trrep4(tuple) _rrep4 tuple
#define _rrep1(n) for(lint i=n-1;i>=0;--i)
#define _rrep2(i,n) for(lint i=n-1;i>=0;--i)
#define _rrep3(i,a,b) for(lint i=b-1;i>=a;--i)
#define _rrep4(i,a,b,c) for(lint i=a+(b-a-1)/c*c;i>=a;i-=c)
#define rep(...) _vcppunko4((__VA_ARGS__,_trep4,_trep3,_trep2,_rep1))((__VA_ARGS__))
#define per(...) _vcppunko4((__VA_ARGS__,_trrep4,_trrep3,_trrep2,_rrep1))((__VA_ARGS__))
#define each(c) for(auto &e:c)

//io
template<class T>
istream& operator>>(istream& is,vector<T>& vec);
template<class T,size_t size>
istream& operator>>(istream& is,array<T,size>& vec);
template<class T,class L>
istream& operator>>(istream& is,pair<T,L>& p);
template<class T>
ostream& operator<<(ostream& os,vector<T>& vec);
template<class T,class L>
ostream& operator<<(ostream& os,pair<T,L>& p);
template<class T>
istream& operator>>(istream& is,vector<T>& vec){ for(T& x: vec) is>>x;return is; }
template<class T,class L>
istream& operator>>(istream& is,pair<T,L>& p){ is>>p.first;is>>p.second;return is; }
template<class T,class L>
ostream& operator<<(ostream& os,pair<T,L>& p){ os<<p.first<<" "<<p.second;return os; }
template<class T>
ostream& operator<<(ostream& os,vector<T>& vec){ os<<vec[0];rep(i,1,vec.size())os<<' '<<vec[i];return os; }
template<class T>
ostream& operator<<(ostream& os,deque<T>& deq){ os<<deq[0];rep(i,1,deq.size())os<<' '<<deq[i];return os; }


inline void in(){}
template <class Head,class... Tail>
inline void in(Head&& head,Tail&&... tail){ cin>>head;in(move(tail)...); }
inline bool out(){ return(cout<<'\n',0); }
template <class T>
inline bool out(T t){ return(cout<<t<<'\n',0); }
template <class Head,class... Tail>
inline bool out(Head head,Tail... tail){ cout<<head<<' ';out(move(tail)...);return 0; }
template <class T>
inline void alloc(T &c,lint s){ rep(c.size())c[i].resize(s); }
#define alc alloc

//TA
#define lin(...) lint __VA_ARGS__;in(__VA_ARGS__)
#define stin(...) string __VA_ARGS__;in(__VA_ARGS__)
#define vin(type,name,size) vector<type> name(size);in(name)
#define fi e.first
#define se e.second
#define YES(c) cout<<((c)?"YES\n":"NO\n"),0
#define Yes(c) cout<<((c)?"Yes\n":"No\n"),0
#define POSSIBLE(c) cout<<((c)?"POSSIBLE\n":"IMPOSSIBLE\n"),0
#define Possible(c) cout<<((c)?"Possible\n":"Impossible\n"),0
#define o(p) cout<<p<<endl,0
#define sp(p) cout<<p<<" "
#define no(p) cout<<p
#define chmin(l,r) l=min(l,r)
#define chmax(l,r) l=max(l,r)
#define ve(type) vector<type>

//other
#ifdef __ENV_TQK__
#define deb(p) cout<<p<<endl,0
#else
#define deb(p) 0
#endif
struct Fastio{
	Fastio(){
		cin.tie(0),cout.tie(0);
		ios::sync_with_stdio(0);
		cout<<fixed<<setprecision(10);
	}
} __fastio;

//mint
#define md_tmp template<uint_fast64_t md=1000000007>
md_tmp class mint{
	using u64=uint_fast64_t;

public:
	u64 a;

	constexpr mint(const u64 x=0) noexcept: a(x%md){}
	constexpr u64 &value() noexcept{ return a; }
	constexpr const u64 &value() const noexcept{ return a; }
	constexpr mint operator+(const mint rhs) const noexcept{
		return mint(*this)+=rhs;
	}
	constexpr mint operator-(const mint rhs) const noexcept{
		return mint(*this)-=rhs;
	}
	constexpr mint operator*(const mint rhs) const noexcept{
		return mint(*this)*=rhs;
	}
	constexpr mint operator^(const lint rhs) const noexcept{
		return mint(*this)^=rhs;
	}
	constexpr mint operator/(const mint rhs) const noexcept{
		return mint(*this)/=rhs;
	}
	constexpr mint &operator+=(const mint rhs) noexcept{
		a+=rhs.a;
		if(a>=md)a-=md;
		return *this;
	}
	constexpr mint &operator-=(const mint rhs) noexcept{
		if(a<rhs.a)a+=md;
		a-=rhs.a;
		return *this;
	}
	constexpr mint &operator*=(const mint rhs) noexcept{
		a=a*rhs.a%md;
		return *this;
	}
	constexpr mint &operator^=(const lint rhs) noexcept{
		if(!rhs)return *this=1;
		u64 exp=rhs-1;
		mint base=this->a;
		while(exp){
			if(exp&1)*this*=base;
			base*=base;
			exp>>=1;
		}
		return *this;
	}
	constexpr mint &operator/=(const mint rhs) noexcept{
		a=(*this*(rhs^(md-2))).a;
		return *this;
	}
};
md_tmp istream& operator>>(istream& os,mint<md>& m){
	os>>m.a,m.a%=md;
	return os;
}
md_tmp ostream& operator<<(ostream& os,const mint<md>& m){
	return os<<m.a;
}
md_tmp mint<md> ncr(lint n,lint r){
	if(n<r||n<0||r<0)return mint<md>(0);
	mint<md>ncr_res=1,ncr_div=1;
	rep(r)ncr_res*=(n-i),ncr_div*=(r-i);
	return ncr_res/ncr_div;
}
#ifndef _AOJ_
mint<> operator""m(unsigned long long n){ return mint<>(n); }
mint<998244353> operator""m9(unsigned long long n){ return mint<998244353>(n); }
mint<1000003> operator""m3(unsigned long long n){ return mint<1000003>(n); }
#endif



//const
#define linf 1152921504606846976
#define inf linf
#define MAXN 330
#define md_1e9_7 1000000007
#define md_998244353 998244353
#define pi 3.14159265358979323846
//#define mod md_1e9_7
const int d4[5]={0,1,0,-1,0};

//main------

class P{ public:lint f,s; P(lint a,lint b):f(a),s(b){}; P():f(0),s(0){}; };
istream& operator>>(istream& os,P& p){ os>>p.f>>p.s;return os; }
bool operator<(const P& l,const P& r){ return(l.f-r.f?l.f<r.f:l.s<r.s); }
bool operator>(const P& l,const P& r){ return(l.f-r.f?l.f>r.f:l.s>r.s); }

template< typename T >
struct Dinic{
	struct edge{
		int to;
		T cap;
		int rev;
		edge(int t,T c,int r):to(t),cap(c),rev(r){}
	};

	const T INF;
	vector< vector< edge > > graph;
	vector< T > min_cost;
	vector< int > iter;

	Dinic(int n): INF(numeric_limits< T >::max()){
		graph.resize(n);
	}

	void add_edge(int from,int to,T cap){
		graph[from].push_back(edge( to,cap,(int)graph[to].size() ));
		graph[to].push_back(edge( from,0,(int)graph[from].size() - 1 ));
	}

	bool bfs(int s,int t){
		min_cost.assign(graph.size(),-1);
		queue< int > que;
		min_cost[s] = 0;
		que.push(s);
		while(!que.empty()){
			int p = que.front();
			que.pop();
			for(auto &e : graph[p]){
				if(e.cap > 0 && min_cost[e.to] == -1){
					min_cost[e.to] = min_cost[p] + 1;
					que.push(e.to);
				}
			}
		}
		return (min_cost[t] != -1);
	}

	T dfs(int idx,const int t,T flow){
		if(idx == t) return (flow);
		for(int &i = iter[idx]; i < graph[idx].size(); i++){
			edge &e = graph[idx][i];
			if(e.cap > 0 && min_cost[idx] < min_cost[e.to]){
				T d = dfs(e.to,t,min(flow,e.cap));
				if(d > 0){
					e.cap -= d;
					graph[e.to][e.rev].cap += d;
					return (d);
				}
			}
		}
		return (0);
	}

	T max_flow(int s,int t){
		T flow = 0;
		while(bfs(s,t)){
			iter.assign(graph.size(),0);
			T f = 0;
			while((f = dfs(s,t,INF)) > 0){
				flow += f;
			}
		}
		return (flow);
	}
};


int main(){
	lint n;in(n);
	vector<P> a(n);
	string s;
	rep(n)in(s),a[i]={s[0]-'a',s[s.size()-1]-'a'};
	rep(26){
		Dinic<lint> G(27);
		lint ideg=0,odeg=0;
		each(a){
			if(e.s==i)++ideg;
			if(e.f==i){
				++odeg;
				G.add_edge(26,e.s,1l);
			} else{
				G.add_edge(e.f,e.s,1l);
			}
		}
		if(ideg&&ideg>=odeg&&G.max_flow(26,i)==odeg)o(char('a'+i));
	}
}

//sub-EOF


Measured memory_bytes: 4376