#include<stdio.h>
#include<math.h>
int main()
{
    double a, b, c, s, p;
    scanf("%lf %lf %lf", &a, &b, &c);
    p=(a+b+c)*0.5;
    s = sqrt(p * (p - a) + p * (p - b) + p * (p - c));
    printf("%.1lf", s);
    return 0;
}