https://www.acmicpc.net/problem/1157
import java.io.*;
import java.util.*;
class Main {
public static void main(String[] args) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
int cnt[] = new int[26];
String s = br.readLine();
for (int i = 0; i < s.length(); i++) {
if (s.charAt(i) >= 'a' && s.charAt(i) <= 'z') {
cnt[s.charAt(i) - 'a']++;
} else {
cnt[s.charAt(i) - 'A']++;
}
}
int max = -1;
char c = '?';
for (int i = 0; i < 26; i++) {
if (cnt[i] > max) {
max = cnt[i];
c = (char) (i + 65);
} else if (cnt[i] == max) {
c = '?';
}
}
System.out.println(c);
}
}