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