#include <bits/stdc++.h>
using namespace std;
int main() {
  string str;
  cin >> str;
  int k = 0;
  long long int n = 0;
  long long int m = 0;
  for (long long int i = 0; i < 100000; i++) {
    string substr = str.substr(k, 1);
    if (substr == "0") {
      n = n + 1;
    } else {
      m = m + 1;
    }
    k = k + 1;
  }
  if (n >= m) {
    cout << m * 2 << endl;
  } else {
    cout << n * 2 << endl;
  }
  return 0;
}
