#include <stdio.h>

int main(){
  long long a,b,ans;

  scanf("%lld%lld",&a,&b);

  ans=a*b/2-1+(a%2)*(b%2);

  if(a==b)
    if(a%2)
    ans=a/2*((a+1)/2);
    else
    ans=a*b/4-1;
  

if(ans<0)
  ans=0;



  printf("%lld\n",ans);

  return 0;
}