#include <stdio.h>
#include <math.h>
/*This program does not use #define to solve the problem*/
int main()
{
    double a,b,c,p;
    scanf("%lf %lf %lf",&a,&b,&c);
    p=(a+b+c)/2;
    printf("%.3lf\n",sqrt(p*(p-a)*(p-b)*(p-c)));
    return 0;
}