#include <stdio.h>
int have_letter(int s[]) {
int flag = 0;
for (int i = 0; i < 26; i++) {
if (s[i] > 0) {
flag = 1;
}
}
return flag;
}
void result(char s[]) {
int i = 0;
int counter[26] = {0};
int number;
while (s[i] != '\0') {
if (s[i] >= 'A' && s[i] <= 'Z') {
number = s[i] - 'A';
counter[number]++;
} else if (s[i] >= 'a' && s[i] <= 'z') {
number = s[i] - 'a';
counter[number]++;
}
i++;
}
int flag = have_letter(counter);
if (!flag) {
printf("-");
} else {
int max = 0;
for (int j = 0; j < 26; j++) {
if (counter[max] < counter[j]) {
max = j;
}
}
printf("%c", max + 'a');
}
}
int main() {
char a[1001];
scanf("%[^\n]", a);
result(a);
return 0;
}