#include <bits/stdc++.h>
using namespace std;
int main() {
  char s;
  long long int tmp = 10 ^ 5, i = 0;
  char a[tmp];
  while (cin >> s) {
    a[i] = s;
    i++;
  }
  long long int N = i;
  long long int C0 = 0, C1 = 0;
  for (i = 0; i < N; i++) {
    if (a[i] == '0')
      C0++;
    else if (a[i] == '1')
      C1++;
  }
  cout << 2 * min(C0, C1) << endl;
  return 0;
}
