#include <bits/stdc++.h>
using namespace std;
int main() {
  int z=0, o=0;
  char j;
  while(scanf("%c", &j) != EOF){
    if(j == '0') z++;
    else if(j == '1') o++;
  }
  cout << 2*min(z, o) << endl;
  return 0;
}