#include <stdio.h>
#include <stdlib.h>

int main()
{

    double S,a,b,c,p;
    scanf("%d %d %d",&a,&b,&c);
     p=(a+b+c)/2;
     printf("%.1lf\n",S=sqrt(p*(p-a)*(p-b)*(p-c)));
return 0;

}