# 143 A

a, b = map(int, input().split())

if a >= (b * 2):
    print(a - (b * 2))
elif a < (b * 2):
    print(0) a, b = map(int, input().split()) a map(int, input().split()) map int input().split() ().split () input split b if a >= (b * 2):
    print(a - (b * 2))
elif a < (b * 2):
    print(0) a >= (b * 2) a b * 2 b 2 print(a - (b * 2)) print a - (b * 2) a b * 2 b 2 elif a < (b * 2):
    print(0) a < (b * 2) a b * 2 b 2 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
# 143 A

a, b = map(int, input().split())

if a >= (b * 2):
    print(a - (b * 2))
elif a < (b * 2):
    print(0)