
a,b=map(int,input().split())
if b>=a:
    print('0')
elif a-2*b >= 0:
    print(a-2*b)
else:
    print('0') a,b=map(int,input().split()) a map(int,input().split()) map int input().split() ().split () input split b if b>=a:
    print('0')
elif a-2*b >= 0:
    print(a-2*b)
else:
    print('0') b>=a b a print('0') print '0' elif a-2*b >= 0:
    print(a-2*b) a-2*b >= 0 a-2*b a 2*b 2 b 0 print(a-2*b) print a-2*b a 2*b 2 b print('0') print '0' b=map(int,input().split()) map(int,input().split()) b a,b=map(int,input().split()) map(int,input().split()) a

a,b=map(int,input().split())
if b>=a:
    print('0')
elif a-2*b >= 0:
    print(a-2*b)
else:
    print('0')