For a better world

#include <iostream>
#include <string>
#include <algorithm>
using namespace std;

int main(void)
{
	string s;
	cin >> s;
	int max = 0; 
	int temp = 0;
	bool flag = false;
	int arr[26] = { 0, };
	
	transform(s.begin(), s.end(), s.begin(), ::toupper); // 소문자화
	
	for (int i = 0; i < s.length(); i++)
		arr[int(s[i] - 65)] += 1;

	for (int i = 0; i < 26; i++)
	{
		if (arr[i] > max)
		{
			max = arr[i];
			temp = i;
		}
	}

	for (int i = 0; i < 26; i++)
	{
		if ((max == arr[i]) and (i != temp))
			flag = true;
	}

	if (flag)
		std::cout << "?" << endl;
	else
		std::cout << char(temp+65) << endl;

	return 0;
}