#include <stdio.h>

int main() 

{
  //활동3-1//
   /* int a,b;
    scanf("%d",&a);
    scanf("%d",&b);
    printf("%d+%d=%d\n",a,b,a+b);
    printf("%d-%d=%d\n",a,b,a-b);
    printf("%d*%d=%d\n",a,b,a*b);
    printf("%d/%d=%d\n",a,b,a/b);
    printf("%d%%%d=%d\n",a,b,a%b); */

    //활동3-2//
    float a,b;
    scanf("%f",&a);
    scanf("%f",&b);
    printf("%.1f",a*b/2);

    
    
    return 0;
}