import math

def main():
    l, k = map(int, input().split())
    if l/2 > k and l/2 % k == 0:
        print(round(((l/2)//k)*k)-k)
    else:
        if l/2 > k:
            print(round(((l/2)//k)*k))
        else:
            print(0)

if __name__ == '__main__':
    main()