# coding: utf-8
#yuki_84

r,c=map(int,raw_input().split())

if r%2 and c%2:
 if r==1 and c==1:
  res=0
 elif r==c:
  res=(r*c+1)/4-1
 else:
  res=(r*c+1)/2-1
else:
 if r==2 and c==2:
  res=0
 elif r==c:
  res=(r*c+1)/4-1
 else:
  res=(r*c+1)/2-1

print res