import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) throws Exception {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String s = arr01[0];
        int l = s.length();

        int cnt1 = 0;
        int cnt2 = 0;
        for (int i = 0; i < l; i++) {
            if (s.substring(i, i + 1).equals("0")) {
                cnt1 += 1;
            } else {
                cnt2 += 1;
            }
        }

        System.out.println(l - Math.abs(cnt1 - cnt2));

    }

} import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io import java.util.ArrayList; ArrayList java.util import java.util.List; List java.util public class Main {

    public static void main(String[] args) throws Exception {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String s = arr01[0];
        int l = s.length();

        int cnt1 = 0;
        int cnt2 = 0;
        for (int i = 0; i < l; i++) {
            if (s.substring(i, i + 1).equals("0")) {
                cnt1 += 1;
            } else {
                cnt2 += 1;
            }
        }

        System.out.println(l - Math.abs(cnt1 - cnt2));

    }

} Main public static void main(String[] args) throws Exception {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String s = arr01[0];
        int l = s.length();

        int cnt1 = 0;
        int cnt2 = 0;
        for (int i = 0; i < l; i++) {
            if (s.substring(i, i + 1).equals("0")) {
                cnt1 += 1;
            } else {
                cnt2 += 1;
            }
        }

        System.out.println(l - Math.abs(cnt1 - cnt2));

    } main {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String s = arr01[0];
        int l = s.length();

        int cnt1 = 0;
        int cnt2 = 0;
        for (int i = 0; i < l; i++) {
            if (s.substring(i, i + 1).equals("0")) {
                cnt1 += 1;
            } else {
                cnt2 += 1;
            }
        }

        System.out.println(l - Math.abs(cnt1 - cnt2));

    } InputStreamReader is = new InputStreamReader(System.in); is new InputStreamReader(System.in) BufferedReader reader = new BufferedReader(is); reader new BufferedReader(is) List<String> mylist = new ArrayList<>(); mylist new ArrayList<>() String strinput = reader.readLine(); strinput reader.readLine() reader.readLine reader while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        } strinput != null strinput null {

            mylist.add(strinput);
            strinput = reader.readLine();

        } mylist.add(strinput) mylist.add mylist strinput strinput = reader.readLine() strinput reader.readLine() reader.readLine reader String arr01[] = mylist.get(0).split(" "); arr01 mylist.get(0).split(" ") mylist.get(0).split mylist.get(0) mylist.get mylist 0 " " String s = arr01[0]; s arr01[0] arr01 0 int l = s.length(); l s.length() s.length s int cnt1 = 0; cnt1 0 int cnt2 = 0; cnt2 0 for (int i = 0; i < l; i++) {
            if (s.substring(i, i + 1).equals("0")) {
                cnt1 += 1;
            } else {
                cnt2 += 1;
            }
        } int i = 0; int i = 0; i 0 i < l i l i++ i++ i {
            if (s.substring(i, i + 1).equals("0")) {
                cnt1 += 1;
            } else {
                cnt2 += 1;
            }
        } {
            if (s.substring(i, i + 1).equals("0")) {
                cnt1 += 1;
            } else {
                cnt2 += 1;
            }
        } if (s.substring(i, i + 1).equals("0")) {
                cnt1 += 1;
            } else {
                cnt2 += 1;
            } s.substring(i, i + 1).equals("0") s.substring(i, i + 1).equals s.substring(i, i + 1) s.substring s i i + 1 i 1 "0" {
                cnt1 += 1;
            } cnt1 += 1 cnt1 1 {
                cnt2 += 1;
            } cnt2 += 1 cnt2 1 System.out.println(l - Math.abs(cnt1 - cnt2)) System.out.println System.out System System.out l - Math.abs(cnt1 - cnt2) l Math.abs(cnt1 - cnt2) Math.abs Math cnt1 - cnt2 cnt1 cnt2 String[] args args public class Main {

    public static void main(String[] args) throws Exception {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String s = arr01[0];
        int l = s.length();

        int cnt1 = 0;
        int cnt2 = 0;
        for (int i = 0; i < l; i++) {
            if (s.substring(i, i + 1).equals("0")) {
                cnt1 += 1;
            } else {
                cnt2 += 1;
            }
        }

        System.out.println(l - Math.abs(cnt1 - cnt2));

    }

} public class Main {

    public static void main(String[] args) throws Exception {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String s = arr01[0];
        int l = s.length();

        int cnt1 = 0;
        int cnt2 = 0;
        for (int i = 0; i < l; i++) {
            if (s.substring(i, i + 1).equals("0")) {
                cnt1 += 1;
            } else {
                cnt2 += 1;
            }
        }

        System.out.println(l - Math.abs(cnt1 - cnt2));

    }

} Main
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) throws Exception {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String s = arr01[0];
        int l = s.length();

        int cnt1 = 0;
        int cnt2 = 0;
        for (int i = 0; i < l; i++) {
            if (s.substring(i, i + 1).equals("0")) {
                cnt1 += 1;
            } else {
                cnt2 += 1;
            }
        }

        System.out.println(l - Math.abs(cnt1 - cnt2));

    }

}
