#include <bits/stdc++.h>
int main(){
	double a,b,c,p,s;
	scanf("%lf%lf%lf",&a,&b,&c);
	p=(a+b+c)/2;
	s=sqrt(p*(p-a)*(p-b)*(p-c));
	printf("%.1lf\n",s);
	return 0;
}

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