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