
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Map;
import java.util.Queue;
import java.util.Set;
import java.util.StringTokenizer;

public class Main {
	// AtCoder Beginner Contest 120
	// https://atcoder.jp/contests/abc120/tasks
	public static void main(String[] args) {
		FastReader sc = new FastReader();
		String s = sc.next();
		LinkedList<Integer> stack = new LinkedList<>();
		int res = 0;
		
		for (char c: s.toCharArray()) {
			int code = c - '0';
			
			if (!stack.isEmpty() && (code ^ (stack.peek())) == 1) {
				stack.pop();
				res += 2;
			} else {
				stack.push(c-'0');
			}
		}
		
		System.out.println(res);
	}
	
	static class FastReader { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
}

class Point {
	int x;
	int y;
	public Point(int x, int y) {
		this.x = x;
		this.y = y;
	}
} import java.io.BufferedReader; BufferedReader java.io import java.io.IOException; IOException java.io import java.io.InputStreamReader; InputStreamReader java.io import java.util.Arrays; Arrays java.util import java.util.HashMap; HashMap java.util import java.util.HashSet; HashSet java.util import java.util.LinkedList; LinkedList java.util import java.util.Map; Map java.util import java.util.Queue; Queue java.util import java.util.Set; Set java.util import java.util.StringTokenizer; StringTokenizer java.util public class Main {
	// AtCoder Beginner Contest 120
	// https://atcoder.jp/contests/abc120/tasks
	public static void main(String[] args) {
		FastReader sc = new FastReader();
		String s = sc.next();
		LinkedList<Integer> stack = new LinkedList<>();
		int res = 0;
		
		for (char c: s.toCharArray()) {
			int code = c - '0';
			
			if (!stack.isEmpty() && (code ^ (stack.peek())) == 1) {
				stack.pop();
				res += 2;
			} else {
				stack.push(c-'0');
			}
		}
		
		System.out.println(res);
	}
	
	static class FastReader { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
} Main // AtCoder Beginner Contest 120
	// https://atcoder.jp/contests/abc120/tasks
	public static void main(String[] args) {
		FastReader sc = new FastReader();
		String s = sc.next();
		LinkedList<Integer> stack = new LinkedList<>();
		int res = 0;
		
		for (char c: s.toCharArray()) {
			int code = c - '0';
			
			if (!stack.isEmpty() && (code ^ (stack.peek())) == 1) {
				stack.pop();
				res += 2;
			} else {
				stack.push(c-'0');
			}
		}
		
		System.out.println(res);
	} main {
		FastReader sc = new FastReader();
		String s = sc.next();
		LinkedList<Integer> stack = new LinkedList<>();
		int res = 0;
		
		for (char c: s.toCharArray()) {
			int code = c - '0';
			
			if (!stack.isEmpty() && (code ^ (stack.peek())) == 1) {
				stack.pop();
				res += 2;
			} else {
				stack.push(c-'0');
			}
		}
		
		System.out.println(res);
	} FastReader sc = new FastReader(); sc new FastReader() String s = sc.next(); s sc.next() sc.next sc LinkedList<Integer> stack = new LinkedList<>(); stack new LinkedList<>() int res = 0; res 0 for (char c: s.toCharArray()) {
			int code = c - '0';
			
			if (!stack.isEmpty() && (code ^ (stack.peek())) == 1) {
				stack.pop();
				res += 2;
			} else {
				stack.push(c-'0');
			}
		} char c s.toCharArray() s.toCharArray s {
			int code = c - '0';
			
			if (!stack.isEmpty() && (code ^ (stack.peek())) == 1) {
				stack.pop();
				res += 2;
			} else {
				stack.push(c-'0');
			}
		} {
			int code = c - '0';
			
			if (!stack.isEmpty() && (code ^ (stack.peek())) == 1) {
				stack.pop();
				res += 2;
			} else {
				stack.push(c-'0');
			}
		} int code = c - '0'; code c - '0' c '0' if (!stack.isEmpty() && (code ^ (stack.peek())) == 1) {
				stack.pop();
				res += 2;
			} else {
				stack.push(c-'0');
			} !stack.isEmpty() && (code ^ (stack.peek())) == 1 !stack.isEmpty() stack.isEmpty() stack.isEmpty stack (code ^ (stack.peek())) == 1 (code ^ (stack.peek())) code (stack.peek()) stack.peek stack 1 {
				stack.pop();
				res += 2;
			} stack.pop() stack.pop stack res += 2 res 2 {
				stack.push(c-'0');
			} stack.push(c-'0') stack.push stack c-'0' c '0' System.out.println(res) System.out.println System.out System System.out res String[] args args static class FastReader { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } FastReader BufferedReader br; br StringTokenizer st; st public FastReader() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } FastReader { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } br = new BufferedReader(new InputStreamReader(System.in)) br new BufferedReader(new InputStreamReader(System.in)) String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } next { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } st == null || !st.hasMoreElements() st == null st null !st.hasMoreElements() st.hasMoreElements() st.hasMoreElements st { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } IOException  e { 
                    e.printStackTrace(); 
                } e.printStackTrace() e.printStackTrace e { 
                    st = new StringTokenizer(br.readLine()); 
                } st = new StringTokenizer(br.readLine()) st new StringTokenizer(br.readLine()) return st.nextToken(); st.nextToken() st.nextToken st int nextInt() { 
            return Integer.parseInt(next()); 
        } nextInt { 
            return Integer.parseInt(next()); 
        } return Integer.parseInt(next()); Integer.parseInt(next()) Integer.parseInt Integer next() next long nextLong() { 
            return Long.parseLong(next()); 
        } nextLong { 
            return Long.parseLong(next()); 
        } return Long.parseLong(next()); Long.parseLong(next()) Long.parseLong Long next() next double nextDouble() { 
            return Double.parseDouble(next()); 
        } nextDouble { 
            return Double.parseDouble(next()); 
        } return Double.parseDouble(next()); Double.parseDouble(next()) Double.parseDouble Double next() next String nextLine() { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } nextLine { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } String str = ""; str "" try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } IOException e { 
                e.printStackTrace(); 
            } e.printStackTrace() e.printStackTrace e { 
                str = br.readLine(); 
            } str = br.readLine() str br.readLine() br.readLine br return str; str class Point {
	int x;
	int y;
	public Point(int x, int y) {
		this.x = x;
		this.y = y;
	}
} Point int x; x int y; y public Point(int x, int y) {
		this.x = x;
		this.y = y;
	} Point {
		this.x = x;
		this.y = y;
	} this.x = x this.x this this.x x this.y = y this.y this this.y y int x x int y y public class Main {
	// AtCoder Beginner Contest 120
	// https://atcoder.jp/contests/abc120/tasks
	public static void main(String[] args) {
		FastReader sc = new FastReader();
		String s = sc.next();
		LinkedList<Integer> stack = new LinkedList<>();
		int res = 0;
		
		for (char c: s.toCharArray()) {
			int code = c - '0';
			
			if (!stack.isEmpty() && (code ^ (stack.peek())) == 1) {
				stack.pop();
				res += 2;
			} else {
				stack.push(c-'0');
			}
		}
		
		System.out.println(res);
	}
	
	static class FastReader { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
} public class Main {
	// AtCoder Beginner Contest 120
	// https://atcoder.jp/contests/abc120/tasks
	public static void main(String[] args) {
		FastReader sc = new FastReader();
		String s = sc.next();
		LinkedList<Integer> stack = new LinkedList<>();
		int res = 0;
		
		for (char c: s.toCharArray()) {
			int code = c - '0';
			
			if (!stack.isEmpty() && (code ^ (stack.peek())) == 1) {
				stack.pop();
				res += 2;
			} else {
				stack.push(c-'0');
			}
		}
		
		System.out.println(res);
	}
	
	static class FastReader { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
} Main

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Map;
import java.util.Queue;
import java.util.Set;
import java.util.StringTokenizer;

public class Main {
	// AtCoder Beginner Contest 120
	// https://atcoder.jp/contests/abc120/tasks
	public static void main(String[] args) {
		FastReader sc = new FastReader();
		String s = sc.next();
		LinkedList<Integer> stack = new LinkedList<>();
		int res = 0;
		
		for (char c: s.toCharArray()) {
			int code = c - '0';
			
			if (!stack.isEmpty() && (code ^ (stack.peek())) == 1) {
				stack.pop();
				res += 2;
			} else {
				stack.push(c-'0');
			}
		}
		
		System.out.println(res);
	}
	
	static class FastReader { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine()); 
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int nextInt() { 
            return Integer.parseInt(next()); 
        } 
  
        long nextLong() { 
            return Long.parseLong(next()); 
        } 
  
        double nextDouble() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try{ 
                str = br.readLine(); 
            } catch (IOException e) { 
                e.printStackTrace(); 
            } 
            return str; 
        } 
    } 
}

class Point {
	int x;
	int y;
	public Point(int x, int y) {
		this.x = x;
		this.y = y;
	}
}