#include <bits/stdc++.h>
int main() {
  int i = 0;
  int length = 0;
  int count1 = 0;
  int count2 = 0;
  char str[256];
  scanf("%s", &str);
  length = strlen(str);
  for (i = 0; i < length - 1; i++) {
    if (str[i] == '1') {
      count1++;
    } else {
      count2++;
    }
  }
  if (count1 > count2) {
    printf("%d", count2 * 2);
  } else {
    printf("%d", count1 * 2);
  }
  return 0;
}
