#include <bits/stdc++.h>
using namespace std;
int main() {
  string S;
  cin >> S;
  int N = S.size();
  int a, b, ans = 0;
  for (int i = 0; i < (int)S.length(); ++i) {
    if (S[i] == '0')
      a++;
    else {
      b++;
    }
  }
  cout << 2 * min(a, b);
}
