#include <bits/stdc++.h>
using namespace std;
int main() {
  string S;
  cin >> S;
  int cnt_0 = 0, cnt_1 = 0;
  for (int i = 0; i < S.size(); i++) {
    if (S.at(i) == '0') {
      cnt_0 += 1;
    } else {
      cnt_1 += 1;
    }
  }
  cout << 2 * min(cnt_0, cnt_1) << endl;
}
