#include <bits/stdc++.h>
char s[100001];
using namespace std;
int main() {
  int c0 = 0, c1 = 0;
  gets(s);
  int n;
  n = strlen(s);
  for (int i = 0; i < n; i++)
    if (s[i] == '0')
      c0++;
    else
      c1++;
  printf("%d", 2 * min(c0, c1));
  return 0;
}
