#include <bits/stdc++.h>
using namespace std;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
const int INF = 999999999;
void readFromInputFile() {}
int main() {
  readFromInputFile();
  string s;
  cin >> s;
  int a, b;
  for (int i = 0; i < (int)(s.size()); i++) {
    if (s[i] == '1') {
      a++;
    } else {
      b++;
    }
  }
  cout << min(a, b) * 2 << endl;
  return 0;
}
