#include <bits/stdc++.h>
using namespace std;
int main() {
  char s[100000];
  scanf("%s", s);
  long n = strlen(s);
  int total0, total1;
  for (long i = 0; i < n; i++) {
    if (s[i] == 48) {
      total0++;
    } else {
      total1++;
    }
  }
  printf("%d\n", min(total0, total1) * 2);
  return 0;
}
