// g++ -std=c++11 a.cpp
#include<iostream>
#include<vector>
#include<string>
#include<algorithm>	
#include<map>
#include<set>
#include<unordered_map>
#include<utility>
#include<cmath>
#include<random>
#include<cstring>
#include<queue>
#include<stack>
#include<bitset>
#include<cstdio>
#include<sstream>
#include<iomanip>
#include<assert.h>
#include<typeinfo>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define FOR(i,a) for(auto i:a)
#define pb push_back
#define all(in) in.begin(),in.end()
#define shosu(x) fixed<<setprecision(x)
#define show1d(v) rep(i,v.size())cout<<" "<<v[i];cout<<endl<<endl;
#define show2d(v) rep(i,v.size()){rep(j,v[i].size())cout<<" "<<v[i][j];cout<<endl;}cout<<endl;
using namespace std;
//kaewasuretyuui
typedef long long ll;
//#define int ll
typedef int Def;
typedef pair<Def,Def> pii;
typedef vector<Def> vi;
typedef vector<vi> vvi;
typedef vector<pii> vp;
typedef vector<vp> vvp;
typedef vector<string> vs;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef pair<Def,pii> pip;
typedef vector<pip>vip;
// #define mt make_tuple
// typedef tuple<int,int,int> tp;
// typedef vector<tp> vt;
template<typename A,typename B>bool cmin(A &a,const B &b){return a>b?(a=b,true):false;}
template<typename A,typename B>bool cmax(A &a,const B &b){return a<b?(a=b,true):false;}
//template<class C>constexpr int size(const C &c){return (int)c.size();}
//template<class T,size_t N> constexpr int size(const T (&xs)[N])noexcept{return (int)N;}
const double PI=acos(-1);
const double EPS=1e-9;
Def inf = sizeof(Def) == sizeof(long long) ? 2e18 : 1e9+10;
int dx[]={0,1,0,-1};
int dy[]={1,0,-1,0};
/*
signed main(){
	int n,m;
	cin>>n>>m;
	int w=sqrt(n)+10;
	vi dp(n+1);
	rep(i,m){
		cin>>a>>b;
		if(b>w){
			if(a!=b-1){
				dp[0]+=a+1;
				dp[1]-=a;
				for(int j=b-a-1;j<n;j+=b){
					dp[j]-=b;
					if(j+1<=n)dp[j+1]+=b;
				}
			}else{
				dp[1]+=1;
				dp[]+=;
			}
		}else{
			
		}
	}
	rep(i,n)dp[i+1]+=dp[i];
	rep(i,n)dp[i+1]+=dp[i];
	rep(i,n){
		int sum=dp[i];
		rep(j,w)sum+=d[j][i%j];
		cout<<sum<<endl;
	}
}
*/


struct edge{int to,cap,rev;};//ikisaki youryou gyakuhen
class MF{//max flow
	public:
	int n;
	vector<vector<edge> >G;//[MAX];
	vi level,iter;//[MAX];
	MF(int size){
		n=size;
		G=vector<vector<edge> >(n);
	}
	void add_edge(int from, int to, int cap){
		edge q={to,cap,int(G[to].size())};
		G[from].push_back(q);
		q={from,0,int(G[from].size()-1)};
		G[to].push_back(q);
	}
	void bfs(int s){
		level=vi(n,-1);
		queue<int>q;
		level[s]=0;
		q.push(s);
		while(!q.empty()){
			int v=q.front();q.pop();
			for(int i=0;i<G[v].size();i++){
				edge &e=G[v][i];
				if(e.cap>0&&level[e.to]<0){
					level[e.to]=level[v]+1;
					q.push(e.to);
				}
			}
		}
	}
	int dfs(int v,int t, int f) {
		if(v==t)return f;
		for(int &i=iter[v];i<G[v].size();i++){
			edge &e=G[v][i];
			if(level[v]>=level[e.to]||e.cap<=0) continue;
			int d =dfs(e.to,t,min(f,e.cap));
			if(d>0){
				e.cap-=d;
				G[e.to][e.rev].cap+=d;
				return d;
			}
		}
		return 0;
	}
	int mf(int s,int t) {//from s to t,ford_fulkerson
		int flow=0;
		while(1){
			bfs(s);
			if(level[t]<0)return flow;
			iter=vi(n);
			int f;
			while((f=dfs(s,t,inf))>0)flow+=f;
		}
	}
};
int main(){
	int n;
	cin>>n;
	vs in(n);
	rep(i,n)cin>>in[i];
	vvi co(26,vi(26));
	vi h(26);
	rep(i,n){
		string s=in[i];
		h[s[s.size()-1]-'a']=1;
		co[s[0]-'a'][s[s.size()-1]-'a']++;
	}

/*	vvi fl(26,vi(26));
	rep(i,26)rep(j,26)if(i!=j){
		MF mf(26);
		rep(x,26)rep(y,26)if(x!=y)
			mf.add_edge(x,y,co[x][y]);
		fl[i][j]=mf.mf(i,j);
	}
	*/
	rep(i,26)if(h[i]){
		int sum=0;
		MF mf(27);
		rep(j,26)rep(k,26)if(j!=i&&j!=k&&i!=k&&co[j][k]){
			mf.add_edge(j,k,co[j][k]);
		}
		rep(j,26){
			if(co[i][j])mf.add_edge(i,j,co[i][j]);
			if(co[j][i])mf.add_edge(j,26,co[j][i]);
			sum+=co[i][j];
		}
		int flow=mf.mf(i,26);
		if(flow==sum)cout<<(char)('a'+i)<<endl;
	}
}









Measured memory_bytes: 4796