#include <bits/stdc++.h>
int main() {
  char str[100000];
  int z = 0, o = 0;
  for (int i = 0; i < 100000; i++) str[i] == '\n';
  scanf("%s", str);
  for (int i = 0; str[i] != '\n'; i++) {
    if (str[i] == '0') z++;
    if (str[i] == '1') o++;
  }
  if (z <= o)
    printf("%d", z * 2);
  else
    printf("%d", o * 2);
  return 0;
}
