
S = input()
work=list(S)
i=0
o_cout=0
z_cout=0
for x in work:
    if(x=='1'):
        o_cout+=1
    else:
        z_cout+=1
    
if(o_cout>z_cout):
    cout=o_cout
else:
    cout=z_cout
    

print(cout*2)