# coding: utf-8
#yuki_84

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

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

print res