int main(){
    string s;
    cin>>s;

    int n = s.length();
    int no0 = 0;
    int no1 = 0;

    rep(i,n){
        if(s[i] == '0') no0++;
        else no1++;
    }

    cout << min(no0,no1)*2 <<endl;

    return 0;
}
