#include <bits/stdc++.h>
int main(void) {
  char s[50];
  int m = 0, n = 0, i;
  scanf("%s", &s);
  for (i = 0; s[i] != '\0'; i++) {
    if (s[i] == '1')
      m++;
    else if (s[i] = '0')
      n++;
  }
  if (n <= m)
    printf("%d\n", n * 2);
  else
    printf("%d\n", m * 2);
  return 0;
}
