#include <cstdio>
#include <algorithm>
using namespace std;
bool isNum(char ch) {
return ch >= '0' && ch <= '9';
}
struct Pair {
int cnt;
char ch;
};
Pair arr[36];
void initArr() {
for (int i = 0; i < 10; i++) {
arr[i].ch = i + '0';
arr[i].cnt = 0;
}
for (int i = 0; i < 26; i++) {
arr[i + 10].ch = i + 'a';
arr[i + 10].cnt = 0;
}
}
bool myCmp(Pair a, Pair b) {
if (a.cnt == b.cnt) {
return a.ch < b.ch;
} else {
return a.cnt > b.cnt;
}
}
int main() {
initArr();
char ch;
while (EOF != scanf("%c", &ch)) {
if (ch == '\n') {
sort(arr, arr + 36, myCmp);
for (int i = 0; i < 36; i++) {
if (arr[i].cnt < 1) {
break;
} else {
printf("%c", arr[i].ch);
}
}
printf("\n");
initArr();
} else {
if (isNum(ch)) {
arr[ch - '0'].cnt++;
} else {
arr[ch - 'a' + 10].cnt++;
}
}
}
return 0;
}