#include <bits/stdc++.h>
long long min(long long a, long long b) {
  if (a < b) return a;
  return b;
}
void solve() {
  char s[100];
  scanf("%s", s);
  long long zero = 0, one = 0;
  for (long long i = 0; i < strlen(s); ++i) {
    if (s[i] == '1')
      ++one;
    else
      ++zero;
  }
  long long ans = min(zero, one);
  ans <<= 1;
  printf("%lld\n", ans);
}
int main() {
  solve();
  return 0;
}
