s = input()
cnt=0
while True:
    for i in range(len(s)//2):
        try: 
            if '10' in s:
                cnt+=2 * s.count('10')
                s=s.replace('10', '') 
        except:
            pass
        try: 
            if '01' in s:
                cnt+=2 * s.count('01')
                s=s.replace('01', '') 
        except:
            pass
    break
print(cnt)