import sys
input=lambda: sys.stdin.readline().rstrip()
r,c=map(int,input().split())
if r==c:
  if r==1:
    print(0)
  elif r%2==0:
    print(r//2*c//2-1)
  else:
    print(((r+1)//2)*(c//2))
else:
  if (r*c)%2==1:
    print(r*c//2)
  else:
    print(r*c//2-1)