from collections import deque                                        
                                                                     
def unification(string):                                             
                                                                     
    n = len(string)                                                  
                                                                     
    if n == 1:                                                       
        return 0                                                     
                                                                     
    q = deque()                                                      
    idx = 2                                                          
    q.appendleft(string[0])                                          
    q.appendleft(string[1])                                          
    count = 0                                                        
                                                                     
    while idx < n:                                                   
        while len(q) > 1 and q[0] != q[1]:                           
            q.pop()                                                  
            q.pop()                                                  
            count += 2                                               
                                                                     
        q.appendleft(string[idx])                                    
        idx += 1                                                     
                                                                     
    return count                                                     
                                                                     
                                                                     
if __name__ == "__main__":                                           
    string = input()                                                 
    print(unification(string))     