#include <iostream>
#include <vector>
 
using namespace std;
 
int main(void)
{
	vector<string> sentences;{
		string temp;
		while(!cin.eof()){
			cin>>temp;
			sentences.push_back(temp);
		}
	}
	vector<int> count(26,0);
 
	for(size_t i=0;i<sentences.size();++i)
		for(size_t j=0;j<sentences[i].length();++j)
			count[static_cast<int>(sentences[i][j])-97]++;
 
	int max=0;
 
	for(int i=0;i<26;++i)
		if(max<count[i])
			max=count[i];
	for(int i=0;i<26;++i)
		if(count[i]==max)
			cout<<static_cast<char>(i+97);
	return 0;
}
 

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