#include <bits/stdc++.h>
int main(void) {
  char n;
  int r, b;
  int i;
  r = 0;
  b = 0;
  while (scanf("%c", &n) != EOF) {
    if (n == '0')
      r++;
    else
      b++;
  }
  printf("%d\n", (r < b ? r : b) * 2);
  return 0;
}
