#include<bits/stdc++.h>
#define debug(x) cerr<<#x<<"="<<(x)<<endl;
using namespace std;
inline void write(int x)
{
	if(x<0)
	{
		putchar('-');
		x=-x;
	}
	if(x>9)
		write(x/10);
	putchar(x%10+'0');
}
inline int read()
{
	int x=0,f=1;
	char c=getchar();
	while(c<'0'||c>'9')
	{
		if(c=='-')
			f=-1;
		c=getchar();
	}
	while(c>='0'&&c<='9')
	{
		x=x*10+c-'0';
		c=getchar();
	}
    return x*f;
}
int s1[50],s2[50][50],s3[50][50][50];
string s[100005];
int main()
{
	//freopen(".in","r",stdin);
	//freopen(".out","w",stdout);
    ios::sync_with_stdio(false);
    int n=read();
    for(int i=1;i<=n;++i)
    {
    	cin>>s[i];
    	if(s[i].size()==1)
    		++s1[s[i][0]-'a'];
    	else if(s[i].size()==2)
    	{
    		++s2[s[i][0]-'a'][s[i][1]-'a'];
		}
		else
		{
			++s3[s[i][0]-'a'][s[i][1]-'a'][s[i][2]-'a'];
		}
	}
    int maxsize=0;
    string maxx="";
    for(int i=0;i<=26;++i)
    {
    	if(s1[i]>maxsize)
    	{
    		maxx=i+'a';
    		maxsize=s1[i];
		}
	}
	for(int i=0;i<=26;++i)
    {
    	for(int j=0;j<=26;++j)
		{
			if(s2[i][j]>maxsize)
			{
				maxx=i+'a';
				maxx+=j+'a';
				maxsize=s2[i][j];
			}
		}
	}
	for(int i=0;i<=26;++i)
    {
    	for(int j=0;j<=26;++j)
		{
			for(int k=0;k<=26;++k)
			{
				if(s3[i][j][k]>maxsize)
				{
					maxx=i+'a';
					maxx+=j+'a';
					maxx+=k+'a';
					maxsize=s3[i][j][k];
			    }
			}
		}
	}
	cout<<maxx<<endl;
    return 0;
}