#include<cmath>
#include<iostream>
#include<cstdio>
using namespace std;
int main(){
double a,b,c;
scanf("%lf%lf%lf",&a,&b,&c);
double p=(a+b+c)/2.0;
double s=sqrt(p*(p-a)*(p-b)*(p-c));
printf("%.1d",s+0.5);
return 0;
}