#include <stdio.h>
#pragma warning(disable: 4996)

int main() {
char arr[1000001];
char alp[26];
int max;
int maxalp;
int count = 0;
int len = 0;

for (int i = 0; i < 26; i++) {
alp[i] = 0;
}

scanf("%s", arr);
len = strlen(arr);

for (int i = 0; i<len; i++) {
if (arr[i] > 96) {
arr[i] -= 32;
}
alp[arr[i] - 65]++;
}

max = alp[0];
maxalp = 0;

for (int i = 1; i < 26; i++) {
if (max < alp[i]) {
max = alp[i];
maxalp = i;
count = 0;
}
else if (max == alp[i]) {
count = -1;
}
}

if (count == 0)printf("%c", maxalp + 65);
else if (count == -1)printf("?");

return 0;
}